The exact number of gallons is important.
Don't mix the different 'breeds' (They aren't 'breeds' - they are different species. A chihuahua and great dane are different breeds of dog but a dog and cat are different species.).
In particular, blue gouramies (one color morph of the three-spot gourami - Trichogaster trichopterus) can be very aggressive. Other color morphs of this gourami are the gold, lavender, platinum and opaline colors. Unless you go for the largest tank and are prepaired to go for a group of 4 or more females, these are all best avoided. Males have longer and more pointed dorsal fins. They also grow slightly larger while the females are plumper, look wider when viewed from above and are deeper-bodied and stockier over-all. These fish grow to around 6".
'Rainbow gouramies' are usualy dwarf gourami color morphs. Dwarf gouramies (Colisa lalia) come in many, many different colors with various different common names such as powder blue, neon blue, red, red honey, rainbow etc. All of these grow to 2" and the females are much more dull in coloration compared to the males. The males also have evidently more 'bulbous' anal and dorsal fins and these are also longer. Females are, again, plumper and slightly smaller when fully grown. While these are generaly peaceful fish, they are also extremely prone to disease and bullying from larger species. They do not make good beginner gouramies and you need to also keep in mind that females can be difficult to get hold of. If you go for a smallish tank, a single male or a trio (1 male, 2 females) would work but you need to make sure the tank is stable and be prepaired to deal with diseases.
Pearl gouramies are my favourite species of fish. They are stunning and often social gouramies. These grow to about 4" or 5" - males being slightly larger than females. Again, females look wider when viewed from above and are generaly less streamlined. Males have extended fin rays and their dorsal fin is considerably longer than the females'. Males also develop a breathtaking dark orange-red coloration around th ventral area whereas females are olive green with white spangles. These will do well in any moderately-sized to large tank in a group of three (1 male, 2 females) or in a larger group where the females out-number the males. These are also quite hardy and peaceful so I'd reccomend these above either of the afformentioned species.
Now about tankmates - almost any non-nippy schooling fish works as long as it doesn't compete with the gouramies for space. This means rasboras, many barbs and most tetras work well. Danios are generaly best avoided unless the tank's quite large because they occupy the same upper layers as the gouramies do. The same applies to hatchetfish. Cories, bristlenose plecs, the majority of small, peaceful loaches (dwarf changed loaches or kuhlies, for rexample) and siamese algae eaters (NOT chinese) are also good tankmates though the algae eaters need a slightly larger tank and are best kept with the larger gourami species as their high activity levels can cause stress to dwarfs. Many peaceful cichlids also work - a pair of keyholes or german blue/gold rams for example. I've also kept american-flag fish with gouramies but take care because these can become surprisingly aggressive when breeding. Platies, swordtails and endler's/guppies also work. Make sure you don't keep guppies with three-spots though as they are then at risk from attack. Similarly, avoid mollies which, for some reason, don't seem to get along with gouramies.
Make sure you research any fish you are considering before you buy them.
Whatever you decide, make sure you cycle your tank BEFORE adding fish. Read the links in my signature - particularly the one about fishless cycling.
