This is realy difficult to answer without knowing which species you're talking about.
With most of the common ones, avoid a male/female pair. trios work well suualy though - 1 male, 2 females.
There are a few that do ok as pairs - such as pearls - but things like dwarfs and honeys, despite being sold as such, don't work well as a pair.
With most, all-female groups work very well also.
All-male groups should generaly be avoided. There are some exceptions provided you offer plenty of space per male; or you can add several females per male for a similar effect. In your size tank, most species would work alright even if they were 2 males.
For a 90 gallon, it's difficult to reccomend a species without knowing what else is in the tank. Having said that, I'd expect you to be after something medium-sized so that it's not lost in the volume. Moonlights, pearls, banded gouramies and thick-lipped gouramies are a handful to look at. Just a ntoe here - do NOT judge them on what they look like at the LFS. instead, look up photos of matrue fish on google.com to get an idea of what they'll look like ocne settled in.
You are likely to also come across three-spots while you're looking around (
trichoagster trichopterus). Unless a group of 4 females is what you are planning (or a single male), avoid these. If you mix males they fight, if you keep too few females they fight, if you keep males and females they breed and attack tankmates. There are several color morphs of this species - gold, blue, cosby, opaline, platinum and lavender being a few. Don't assume them to be different species (a common misconception).
As for whether gouramies like being in groups... Again, that depends. Most female gouramies enjoy other (female) company. Males are territorial and do absolutely fine kept singly. There are a few gouramies that border on 'schooling' when kept in a large group in a large volume (eg: pearls) but there are also a few that, even with the females, socialising is not their strong point (eg: paradisefish and, generaly, three-spots and
bettas).
I think, realy, you need to decide which fish you're interested in before you can get a real answer to your question(s). Perhaps a list of what you already ahve int eh tank could help people make a few suggestions
