yes standards are there for multiple, good reasons.
maybe i've misunderstood...
what you are suggesting is to buy a £200+ 51x chassis kit, throw out the backplane, then re-purpose it into a non interchangeable format, with new, non standard edge connections for use as a seriously heavy valve unit? with limited options and space? like a standard rack chassis but with extra holes.
even if it was 11 slots across, you would need 3 spaces for the psu minimum. unless you want to house the psu externally which would mean trailing the seriously ample HT and heaters from a seperate psu chassis like 51x(not recommended) or an expensive dc-dc converter for each pre and very heavy gauge cable between? particularly if you want different 'flavours'
anyway, you suggest a more modest 8 units size, so this would leave 5 spaces max optimistically(8 with 511) next to your psu. presuming you can fit a full multi stage valve compliment for each of 5 mic pre's for example. plus IO transformers in each slot. still there are maybe a couple of designs you could do that with so lets worry about that later.
then have the option(separate power rails+card edges+connections) for an entirely different (51x) format in the same space as the tube units?
I'd love to see some of the designs you mention to gauge exactly how long you have thought about this. I first assumed you have a tube pre design using a crazy low voltage on the plates and series heaters, which may be feasible from a lofi amateur design POV, ignoring the current per unit suggestions. but then you go on to say-
Though maybe building a 8 space box box just for high voltage tube pres may be a better idea then mixing the two.
I think you might have a point there.
there are so many project designs out there, on this forum alone! in the last few weeks even! have you had a look at some/any of the other posts here?
or is this a new, different idea? I'm still confused ???