The word 'gourami' encompasses a huge range of species. They vary greatly in size, activity levels, requirements and aggression.
Judging by the current inhabitants of your tank, though top dwellers are deffinately the better choice, you are better off with some of the larger or medium-sized gouramies with semi-aggressive or boisterous personalities so that they can handle their tankmates.
The most common top-dwelling gouramies available at LFS are trichogaster trichopterus, trichogaster leeri, trichogaster microlepis, colisa chuna/sota, colisa lalia, colisa fasciata and colisa labiosa.
Trichogaster trichopterus is the 'three-spot' gourami. There are several color morphs of this species - lavender, gold, cosby, opaline, blue and platinum being the ones I can think of off the top of my head. All have identical requirements and temperament and can interbreed freely as they are exactly the same species - just different colors. Three-spots are, IMO, the most aggressive of the group commonly available but they don't get too big (males to 6", females ever so slightly smaller) and are extremely hardy fish and very easy to breed. In a tank like yours (which I think is around 90 gallons), a group of 1 male and 4 or so females would do well. Males are highly territorial and aggressive, however, and it is generaly best to keep only females (provided you don't want to breed). If you do end up with a mixed group and the fish breed, you may well end up with various deaths. It's up to you whether or not you take this risk. As for getting more than one male, avoid this at all cost. Two males, even in a large tank, may fight to the death. It's also better to go for as large a group of individuals in total as possible to spread out aggression but don't get mroe than 7 or so (also keep stocking in mind - I didn't pay close attention to the number of fish you already have or how much space you actualy have left for additions). Also, though the species does come in all those color morphs, it's best not to mix three-spots with other gouramies if you have no prior experience with them as they are very likely to clash.
Next, trichogaster leeri, the pearl gourami, was mentioned by the previous poster. Though these are my favourite fish and I reccomend them highly, they have long fins and a very peaceful demeanour that would put them at risk in a tank, such as yours, containing known fin-nippers or boisterous species. Having said that, perhaps you'll find another tank for them
so I'll add that they are also very hardy fish, relatively easy to breed and extremely beautiful. They get to 5" at their largest and mix well with pretty much any peaceful species - whether gourami or not.
Trichogaster microlepis is the largest of the species I've listed here and the least colorful. They get to 7" and are a silver color with red to yellow ventral fins (depending on condition and sex) and, in the right lighting, show a lilac sheen to their very small (hene the scientific name) scales. These are peaceful fish,d espite their size, but can also handle moderate aggression. Sexing juveniles is difficult as only fin and body shape can truly be used reliably but it doesn't realy matter as mixed groups can co-exist quite happily as long as they have enough room to decide territories. They'll mix with all the other gourami species I've mentioned - even the three-spots - but should be allowed plenty of room to themselves.
Colisa lalia - the dwarf gourami - was also mentioned earlier. Again, I would not reccomend these. They are pretty fish with plenty of variations and character but they are also fragile and easily stressed. Mass production and inbreeding have had the effect, besides producing some wonderful color morphs, of causing the fish to become short-lived and very prone to disease. So, though they make good tankmates for most peaceful fish, they are not a good choice for a tank with even mdoerately boisterous species that will stress them - making an already weak fish even mroe susceptible to disease (which then spreads to tankmates and causes serious problems). They are best kept at a ratio of 2 or more females per male and you'll eb able to differentiate between the two because only the males are the flamboyant colors, the females being drab silver or brown with occasional colored highlights or irridescence.
Colisa chuna/sota is the honey gourami. It is, unfortunately, often confused with the dwarf, but is slightly smaller (1.5" as opposed to 2") and much hardier. They should be kept, like the dwarfs, as trios, but the differences between the genders are less obvious with juveniles (all of which are generaly brownish-cream). Adult males of the wild type (as there are some variations) are golden-yellow and develop a blue-black udnerside when in breeding condition. females retain the juvenile coloration. Again, I'd avoid these because of their size and therefore the liklihood of them being bullied by their larger or more active tankmates.
Colisa labiosa and colisa fasciata are, IMO, the best choices for your tank. They won't bother other fish but can handle mild aggression. They are colorful and they are hardy and neither gets too big. The downside is that they aren't quite as common as the afformentioned species. Colisa labiosa is usualy called the 'thick-lipped' gourami and grows to 3.5". Colisa fasciata is known udner many names - amongst these 'giant', 'banded', 'indian' and 'striped' gourami. Be sure to not confuse this fish with the 'true giant' (osphronemus goramy). Colisa fasciata only gets to 4" maximum, osphronemus sp. get to around 30"! Either colisa fasciata or colisa labiosa can be mixed with each other or with pearl gouramies, moonlight gouramies, honeys or dwarfs. I would avoid putting them in with three-spots but they can handle them if necessary. IMO, a trio of each would do nicely in your tank. Females are a little duller than males but still very attractive.
In general, female gouramies preffer to be in groups. males of all the above species are territorial (though some more than others) and do fine on their own. Still, to get the best coloration and behaviour out of males, females need to be present (especialy apparent in pearls and honeys, which only color up fully when in breeding condition). That's just to answer your question concerning how many of each sex it's a good idea to get.
I hope that has pretty much answered your questions but, like I said before, your 90 gallon may not actualy have enough room for any new additions - please make sure it does before adding anything new. Also, make sure your tank contains plenty of hiding places - especialy tall plants or floating cover. Furthermore, please let us know which, if any, of these species you decide to try. People can then post extra info. on that species and tell you how to sex them in more detail or if they have any special requirements that I did not mention.
