I'm gonna +1 on just about everything Abby stated. There's nothing that can substitute experience and ingenuity. And for the following, i know im gonna get reamed out for some of this stuff.... but it's kinda what I go by. I've been doing this for about 15 years now, and im sure some of my "theory" is bunk, but here goes.
As far as "knowing" what will sound good with what... you need to first "KNOW" your equipment. It cannot be stressed enough that if you're going to be commercially operating a studio, that you know your rig inside, out, through and through. Nothing worse than sitting in the ctrl room and looking like its the first time you're using any of the gear, clients pick up on that immediately and its likely that they'll remember that awkwardness when referring your place to another group. Can't tell you the horror stories that my clients have brought in.
As far as getting to know the equipment, its just time my friend. There are things you can do to get to know the characteristics that are a little "unorthodox" but it MAY help you out a bit (see way below). A basic understanding of the make up of the equipment you have is foremost and will help you make an at least "educated" guess on selection.
For Mics:
First, look to tradition for the equipment. What I mean by this, is when taking a AKG D-112 mic out of the cabinet, know that it's traditionally used as a kick drum mic, or bass mic. That doesn't mean that that's ALL its good for (i love a D-112 on a floor tom, but again, that a VERY bass heavy sound source). But know that, it may not be the best selection for a timid female vocalist.
Second, look at the physical/electronic characteristics of the mic. Know the inherent differences of a condenser mic, dynamic mic, and ribbon mic. A dynamic mic with a HUGE diaphragm is going to react differently than a dynamic with a smaller diaphragm. Look at the frequency response curves that come with every mic that most people throw away along with the registration card and other crap in the box. Those can be really helpful in understanding what the mic is and isn't capable of. Know your polar patterns as well, and what they actually mean and what they can do. This will also help you in positioning the mic the "best" way once in the room as well. Placing an omni directional mic in a corner... might not yield the results you're looking for... but then maybe it will... and knowing how that scenario sounds in advance, helps tremendously.
For mic pre's its almost the same. Know the differences between a tube pre and solid state, know the traditional roles of the pre's as a starting point (but dont be afraid to experiment). Know what the path is internally. Is there input/output iron in there? Is everything going through cheap chineese IC's? What kind of tube(s) are in there?
After getting a handle on the technical aspects of the equipment, take some time to make friends with the equipment (sounds kinda weird, but let me explain this one...)
Just like you can recognize a friend by his/her voice, learn to recognize a mic by it's voice. No matter what pre you put an SM57/58 into, you can (after a while) just TELL its a 57/58. Although mic pres can alter how the sound hits your board or daw or whatever is in between IT and the speakers, there are inherent vocal characteristics that are easily recognizable. It should be noted that, when getting to know the equipment, you shouldn't use compression or EQ (unless you're getting to know a compressor or eq... for now we're talking strictly mic/pre combo). After a while, you can get to know that for a mid rangey tone, a 57 is the sound, for a lower mid rangey tone, perhaps a 421, for a bottom heavy source a beta51 or D-112, nice flat open sound, a royer 121, for some nice upper presence and openness a good large diaphragm condenser...
Again, very general statements, but just starting points I go by. Another side note: knowing the voice of the mic can assist even more when trying to pear down undesirable characteristics of a sound source. For instance, if theres a sound source that has an aweful muddy bassyness to it, choosing a mic that does POORLY in the lower registers may be a good choice to combat the problem (if it's perceived as a problem). The only downfall of this is once you've recorded, anything you didn't get, can't always be put back (easily at least). If you record a bass with a mic whose response doesn't go below 1000hz, dont think you're going to get that missing bottom end from EQ or without some synthesis plugin/hardware help.
I like to think that mic's have voices, and pre's have personality. (they have voices too, to a degree, but as a starting point, lets just keep it simple) By personality im talking being passive (in nature), being agressive, being warm, being cold, that kinda thing. Tube pre's tend to be warming, and have musical harmonic nature to them. Pres that use all IC's tend to be very cold and brittle sound. Pres that use good input/output iron can fall in the middle somewhere depending on the type of iron thats in there. Something like an API pre can change their personality based upon which DOA is in there. Some 2520's can be really aggressive, other DOAs are transparent and really don't add/subtract from the source, some can even be a bit on the warm side. Knowing this personality and the voice of the mic in relation to the sound source is the basis for your selection.
Jazzy female vocalist - I want a nice big warm open sound with a good dynamic range... im going to first use a tube pre, with a large condenser. Maybe the vocalists voice is already kind of warm, maybe I'll go to a 1073 pre if the tube pre is too warming...
Screaming male vocalist - dealing with high spl's, might want to go with a good dynamic mic thats got a nice flat wide curve to it. Preamp, depending on how harsh the vocalist is, I might start with the 1073, crank the input gain to put a little more dirt on the vocal. If the voice is nails on a chalkboard, maybe the tube pre, or a pre thats got a nice soft transparent compressor to round out the edges a bit. (i deal with these a lot in my studio) Some vocalists HAVE to have a mic overloaded to get their "sound" so a 58 that they can cup is perfect. Others (the singer in my band) is a very controlled scream and I can put a nice condenser in front of him with great results.
Guitar cab - everyone goes right to the sm57 first (which i kinda hate the sound of but it has its place). I LOVE the royer 121 into the 1073. For a bit more bottom end, I'll throw a 421 on the same speaker (adjusting for phase as best as possible) into either another 1073 or my Joe Meek for some added flavor and combine the 2. If im feeling "saucy" ill throw a 57 on there as well and blend all 3 to taste. (need more mids/highs, bring up the 57, need more bottom, bring up the 421, need OMG tastey goodness, royer it is.
A long time ago I set up a really good studio monitor in my live room which had a nice full recording pumping through it. (seal's kiss from a rose i believe) set up a mic a few feet away and recorded the mic. Back to the top, switch out the mic, rinse repeat for about 5 mic's. (same pre) Then, went back to listen and switched between tracks... INSTANTLY got the jist of each mic's voice. To hear what was accentuated and what was removed for each mic was a great learning experience. Then, once i found what I thought to be a nice full sounding mic with the least coloration, went back to the top and did it again just subbing out the preamps.
This concludes my rant, hope someone didn't get dumber by reading this.
