IMO, you'd need about 40 gals US to keep 20 cardinals, a trio or pair of dwarfs and a couple of bristlenoses. If you can get a bigger tank, then by all means do. The plants also help decrease the amount of nitrAte in the tank so further increase its capacity bio-load wise (though not when it comes to physical space or oxygen content etc).
Also, the neons, flame, red etc dwarf gouramies are all exactly the same species - the dwarf gourami, colisa lalia - and you can comfortably keep two males in a 20 gallon. Females are not aggressive - only the males are territorial. Honeys are honeys
colisa chuna/sota. The males are also territorial but these are slightly smaller so you can keep more.
The only realy common gourami I would not reccomend keeping more than one male in anything less than a 55 gallon is the three-spot (gold, opalines, platinums, cosby, blue etc are all color morphs of this - trichogaster trichopterus). These fish can realy be aggressive once mature but are also triple the size of dwarfs. There should be several females to a single male as well. And if you want to guarantee they wont fight, I would stick to differently colored males with females that match each - eg: a trio of golds and a trio of blues. I don't know why, but differently colored males tend to show less interest in each other.
Something else I'd like to add - the 'inch per gallon guideline' is meant to apply to, firstly, US gallons (which are less than UK so 40 UK gallons are more than enough) and also, it is meant to apply to small, slim-bodied fish - just like cardinals. So, actualy, it does apply to cardinals.
However, what you need to keep in mind is how a fish is emasured - from the eye to the penduncle. So a cardinal only counts for just under an inch.
What it does not apply to is fish like plecs which are big poop-machines.
Just out of curisity BTW, why do you want 2 bristlenoses? There's nothing realy wrong with this but these plecs are territorial (not realy aggressive though) so, unless you want to get a pair to breed them or something, getting one would actualy be better (and leave you with more room for cardinals or gouramies or any future additions which I guarantee you'll eventualy want).
