This is still a work in progress, but the method I've used to work out my stocking is this: start with the surface area. For a normal 29g that's about 360" sq. Divide by 10 and by 12 to get a range of total fish inches. In this case its between 30 and 36 inches of fish. That is a reasonable range for small slender bodied tropical fish. That should be the adult size, not the current size. Now start to factor in the extras:
Are the fish small and slender bodied?
Add extra for fish that are going to get bigger, or fish that are fuller bodied. My (juvenile) gourami aren't much longer than my rasbora, but have a much greater mass. They are bigger fish -- the rainbows are a similar case.
Are the fish carnivores? Are they notorious waste producers (like plecos)?
The extra waste could overwhelm the tank, even if the numbers work. Add some extra room for fish that are going to produce a lot of waste.
Is there enough room for the fish to turn?
A 55 might be big enough for an Oscar, but is a 10" fish going to be comfortable turning in a 12" wide tank? The tank should be twice as wide as the longest fish.
What are the territorial needs of the fish?
Two male bettas will fit very comfortably in a 5 gallon tank. This is still not a good idea. Bettas may be an extreme example, but other fish also have territorial needs.
My guess is that you're already in the low 30"s (not really sure how big the pleco gets). The corys and the pleco are definitely not slender and the gourami and rams aren't especially slender. The pleco is a notorious waste producer. All totaled up, you're probably effectively in the low 40s or so, so your probably already mildly overstocked. The rams and the gouramis are both territorial and may not be happy about further encroachments on their space. And, what with their mass, you'd really have to count the rainbows as more than 10" -- they would leave you heavily overstocked.
Everybody gets to make their own calls, but I was reading something the other day that made a lot of sense to me on overstocking. It said that by keeping conditions ideal we get to enjoy thriving fish, in full color and behaving "normally". By overstocking we lose those things, and lose the very thing we were looking for in the extra fish.
I'm a little crowded now myself. The new tank is almost finished cycling

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