I like the attitude and it is somewhat bold but then we should challenge ourselves now and then, Nothing wrong with it as long as we do it slowly and with some thought. So my questions might involve how you feel about change if it becomes needed? Part of my doubts might involve the firemouth as they do tend to get more rough as they age and when there are several, there is a much better chance of them wanting to breed, which may/may not create some trouble. Something to look over at your fish as they go along?
All fish may act different in different settings but I found pleco to be a bit too hard on my plants, due to rasping on the leaves. Another small point to be aware of as they may work or you may want to trade them out if you begin to see spots on leaves which look thin/ transparent?
A 90 is a 75 but deeper front to back or is it same footprint and taller? If same footprint as 75, I find no major trouble getting light down to the bottom but that can also be worked around with using different plants, so another option to sort out, right?
I quickly reached a point where pressure CO2 was really needed to move further into the plant game, just to get the far better performance out of the plants. Lights are easy to get too much if we don't also move up on feeding ferts and CO2.
Pressure CO2 can be a bite on the wallet but I never felt the DIY CO2 was something for me, so bit the whole thing.
When first thinking on it, try to look at it as a collection of parts, rather than a single item, even though the ads may make you think of it as a single thing to buy.
Basic idea of a CO2 setup?
A regulator to cut the high pressure (800+PSI) down, a solenoid to act as a gate or switch to turn flow on/off and a needle valve to fine tune/adjust that flow before going through something to measure how much is passing ( bubble counter, perhaps?) and then something at the end to diffuse/mix the CO2 into the tank water.
But the complication comes in when we find that we can actually choose Cadillac or Volkswagon levels on each of the parts, I mention. There is no single Caddie version to buy off the shelf, nor a Volks option as we need to build up what level we want on each step. Study, time, effort, versus big bucks upfront, may be as good description as I can offer.