Exactly as Harpo says, of course! I thought that terminology would be clearer, but hey, now you learned it so you can read any cap value! ;D
The MKS and MKP are both metallized film, the MKS being polyester, the MKP polypropylene (C7 is the same as C5). Either will work in either place, in fact some will argue that you don't "need" any of them, they are just bypassing good quality electrolytic caps. But, it's common practice, dating back to when lytic caps were terrible, so you had to bypass them with a film cap to improve highs, but nowadays if you use Panasonic SU bipolars or NIchicon Muse SE or something like that in the signal path, these bypass caps may be overkill. That said, I always use them and generally I choose polypropylene whenever it's in the signal path (and fits), and MKS for power supply bypassing. Even better is FKP (polypropylene film, non metallized) but they are huge (and more expensive).
Another electrolytic cap that is nice for C6 and C8 is Elna Silmic II, (I'mpretty sure I've used them in some 312, but I know I've used them in some SSLs without bypass caps, and these I did do "blind" listening tests and it was clearly better than some other high end lytics (like nichicon KZ, for one). They are not bipolar, so to use them you must arrange them with the positive lead towards the opamp.
Take all this with a grain of salt, of course, caps are caps after all, but if you really want to know if they make a difference, put them in one channel and keep another without and listen to both. It's fun, you might learn something. Or maybe not, it might be BS. ;D
