Actully, you could keep any kind of gouramis, but it depends on their size of how many you could keep.
Well not ANY... Try fitting an osphronemus in there
Still, there are endless options.
In a 20 gallon, I'd go for just 2 of any of the top-dwelling species or possibly a trio if it's one of the smaller ones.
Alternatively, you could get a trio or 5 or so of any of the small, peaceful, more mid-dwelling species.
But first:
Mollies don't usualy like gouramies much. Getting a couple more would solve this problem by providing a distraction. What sex is your molly? If it's female, get 2 more - either both female or one male, 1 female if you don't mind having fry. If it's male, 2 females is best but you'll get fry.
Also, with the platies, if you have a pair, get one more female. Otherwise it should be ok.
Also, are there any other fish you are particularly interested in? That would narrow down the gourami possibilities because at the moment there are a lot!
Is there any one in particular you would like to have?
Firstly, dwarf gouramies (colisa lalia) were mentioned. However beautiful these are, keep in mind that they are prone to disease. They only get to 2" but the males are still aggressive and territorial towards others of their own kind. If you do go for dwarfs, get 2 males OR 1 male and 2 females. The latter being the better option. Females are sometimes difficult to come by because they don't sell as well as the males as they lack the bright colors.
Also mentioned were three-spot gouramies (trichogaster trichopterus). This is the most aggressive of the gouramies you could comfortably keep in a 20 gallon and not realy reccomended (this is entirely my opinion though) despite their extreme hardiness. A single one might work out. They get to 6". You'll see them under several names - blue, gold, opaline, cosby, platinum, lavender and probably several more. These are just color morphs and are otherwise the exact same fish. They are rather aggressive towards all other gouramies or related fish. Females will mix with other species but are still likely to be bullies.
Also mentioned were pearl gouramies. These are my favourite fish and I would highly reccomend a pair or trio for your tank. However, they are generaly very peaceful towards tankmates and prone to being nipped if kept with the wrong fish. I'd be a little worried about the molly. If you find it some friends to occupy it, the molly might be less of an issue though. They get to 5". Both sexes are absolutely beautiful but it's the males that are completely stunning with their bright orange underside and long fins.
Examples of gouramies you should not get (or are best left avoided) are: Paradisefish (macropodus opercularis that is - and not actualy technicaly a gourami but often called that), chocolate gouramies, licorice gouramies, kissing gouramies, moonlight gouramies, snakeskin gouramies and osphronemus gouramies. These are just the realy common unsuitable species.
Some other common gouramies to consider are:
A trio of thick-lipped gouramies (colisa labiosa - 3.5"). Like the dwarf in many ways but hardier.
A tri0o of banded/indian/striped/giant gouramies (colisa fasciata - 4") Again, very simialr to the dwarf but hardier and also the largest of it's genus - hence the 'giant' name. Don't confuse it with the 'true' giant (osphronemus) gouramies or you'll have major issues later.
A trio of honey gouramies (colisa chuna/sota - 1.5"). Lovely little fish that are often confused with dwarfs. Usualy look dull in LFS but brighten up. Males develop a lovely blue-black underside.
Croaking gouramies (trichopsis vittatus - to 2.5" max. but usualy slightly less). A trio would be nice. These are more a mid-dwelling species and realy do appreciate plants in the aquarium. Have lovely blue eyes and can 'croak'.
Sparkling gouramies (trichopsis pumilus - just barely to over 1"). Great, sparkling, little fish. A group of 3-5 with more females than males is best Can also 'croak'. Downside is that their small size may make them targets for bullies - particularly if you end up with quite large fish int eh tank (eg large adult molly).
Also, some of these species do fine mixed but I think you are best off deciding which, if any, you'd b interested in and then finding out if they would work together (or I'd be sat here all day coming up with every possible combination
).
I hope that's of some help but it would help to give some more specific details concerning your tank and what you're after. Is your tank heavily planted? What else have you considered adding? Do you want something particularly hardy? etc...
And thanks fishwatcher for the compliment
though I'm sure there are plenty of other people here who know more and your advice is just as good
.