If you have your tank already you can start cycling it and planting it already, and decide on the puffer when it is ready to welcome new fish. The tank setup will be similar in both cases, and then you can decide on 1 dwarf, 2 dwarf or a red eye.
Minimal size tanks are indications of course, they are not a hard number under which your fish will die quickly or above it it will thrive as in the wild. That's why it vary, everybody have it's own opinion, past experience and tank setup. What is sure is that larger is always better (except for your wallet maybe), that if the minimum size for species A is 2 times the one for species B, then there is agood chance that you will need a tank 2 times bigger for A than for B if you want to have similar keeping experience. Also, I think 2 tanks with the same water capacity may in fact be quite a different habitat for the fish, depending on it's filter, planting and footprint/length/width.
You will get better advices if you launch your tank and post photos of it before deciding on the puffer....but because you target minimum size, you will never get a consensus of course, in fact it's probably around this size that the advices will be the more varied and thus not really help you. If you planned a tank well under the minimum size, everybody wuld tell you no, and well above everybody would tell you go for it. Here it will be mixed of course
Many hobbyists follow the out-of-date (IMHO & IME) practice of moving fish up in tank size "as they need it". This almost always means that the fish will never equal or exceed full wild size - in other words, the captive fish will be smaller and show different configuration from their largest wild cousins. To me that is not the optimum captive care. I may well QT (quarantine) for a minimum of ~ a month, maximum of 2-3 months before moving it to its permanent home (anticipated lifespans from ~5 to 20 or more years). Almost all of my tanks are basically species tanks, with from one to multiple (schooling fish) of the same species, but may have "workers" which are inverts or other fish species present as workers which do not overlap the primary species' biological niche. I have not done mixed or so-called "community" or mixed species tanks in decades and am unlikely to ever do so again. I do always breed 1-3 species of fish or inverts to "pay forward" for all the fish which I did not handle properly when I was younger and less experienced.
The common "minimum" tank size assumes that that tank will also has the optimum tank care, such as 50% weekly water partials with weekly rinsing of all biological and or mechanical filters, and proper but not excessive feeding. Humans have a hard time doing that practice full time. We go on vacations, get ill or break bones, get backed up at work, involved in family crises, etc. Such real life issues tend to result in less tank care than we should do, and water quality suffers more quickly and to higher levels of pollution in "minimum" sized tanks, while larger tanks suffer less during such periods by simple dilution - their bioload is smaller and thus they have wider margins of safety. I am very big on optomizing margins of safety. They give me more assurance that my fish will not suffer if I am temporarily distracted or unavailable. My wife is not a tank hobbyist - she has her own things to do and care for, and if it is vacation time, we do tend to be together. If there is a health care issue, whichever of us is not directly involved tends to be busy careing for the other one who is directly involved, so that is no help for the wet pets. All of my tank systems are set such that they survive well with alternate day water level checks and occasional topping up with prepared water always avaiable and at hand. Two weeks absene by me requires only alternate day measured feedings and simple refills and prep of extra water. Those systems have had some 30 years easy upkeep by even untrained friends and temporary care-givers. A little planning and testing, with confirmed margins of safety have worked essentially flawlessly, even with extended power outages. Obviously, breeding operations are suspended in extened absences.
If you plan and test aqhead of need, tank care is not a big issue, but "minimum" set-ups are just not adequate IME. Margins of safety do make a huge difference.
