Hey man,
In a 15 Gallon, I would really only run one. I grew one plant each in two 15 gallons pots the last run, I think this is maybe their fourth run. The pots seem to get better with age, my first grow I probably jazzed up a bit by getting carried away with a ferment and that took a while to remedy through just plain watering but it did come back great. I have grown 2 plants in one 15 gallons but you end up having to sort of train them away from each other. Both plants ended up being about the same size combined as a single plant would have been, It seems to me that pot size or available root space dictates the plants' size.
If you have space constraints I would either build a physical bed or buy the smart pot/Geo planter bed and use one like a 3x3 or 4x4, that way you can plant more in there. But if you want to physically move the plants around i would either do the 15 if you are cool with that or maybe a 20 gallon. Anything above 20 gets seriously heavy, for me anyways as I did move mine from outside to inside a tent to finish them off. One 15 gallon pretty much filled my 4x4 tents canopy, really about a true 3x3 as my exhaust is a little bit overkill and sucks the tent in a bit. I should mention that was an outdoor plant that had vegged for months outside ( I LST'd the entire plant, bent over the whole thing and all the side branches became cola's if that makes sense). In a tent you may as well speed it up by growing 2-4 plants in a 15. 4 15 gallon pots will fit in a 4x4 no problem with a bit of space around them to place a foot/heater etc or something.
If you have a 5x5 I personally would go with a 4x4 bed. I think you will run into trouble if you try to max out the physical floor footprint, your canopy can extend passed that 4x4 if you wanted but it would probably make more sense to build a scrog off the 4x4 and have it contained in that square. That way you have some amount of space in there for air flow and movement. If your exhaust is strong at all the tent will pull inwards a bit and your plants will be resting against the tent which seems OK actually but I imagine you are losing some light and possibly inviting mold later on in flower perhaps. You can buy probably a 100 gallon smart pot and put it in there also, the specs will tell you it's diameter. I don't see a huge difference other than the square vs circle thing for spacing. You would have to experiment with plant numbers to find an ideal density. I'm in Canada and we are only "allowed" 4 plants now so i was kinda testing for that. I do fully veg my plants to sexual maturity so maybe you can hammer them in there if you are doing short veg times.
I've been mulling around building a 3x3 bed for my 4x4 tent and just keeping it as a flower room and bringing in new plants to flower in the bed. The main drawback I can see is that you can't move the plants if for whatever reason you have to, I have found that if I can put one outside if it's looking a bit sad or weird that the outdoors clears it right up, that may not be an option for you. I do think that more soil = better and probably easier to maintain, although the no till is actually extremely easy, just make sure to water occasionally and mulch the absolute shit out of the surface and it will stay regulated pretty well on its own, also do not go overboard with top dressing or compost tea the plants will find the nutrients, no worries there.
Edit: I think you could just use your super soil and keep it to recycle, No till is just about keeping the dirt where it is. It will have nutrients in it for sure being super soil, don't be fooled into thinking that the soil is depleted after a run cause it's not the no till recipe or whatever, just top dress is occasionally and keep it mulched and it will become "No till" over time, especially after the first run.
As for yield I can only say that the Hashplant crosses I've grown (OGKB,Lemon Thai indy) seem like they would be good yielders and produced well for me. I tested them in a micro environment so I can't really throw you proper numbers based on your projected grow. Hope you enjoy the soil bed! It's great!