Well the tank is a little on the small side but a trio, as it's heavily planted as well, should be fine. That is, 1 male and 2 females. I wouldn't get any mroe than that though.
They are lovely little fish that get to just barely over an inch and will color up beautifuly. During breeding time, the males can actualy croak just like a croaking gourami. Sexing them can be difficult but, generaly, males are brighter and slightly larger than females. A search of this forum should bring up several more methods of sexing them though if you aren't sure. They aren't picky in my experience but look at their best when fed some live/frozen foods. They have also proven to be quite hardy and have great characters once they settle down. In they beginning they may be a little shy though. Good luck with them! I'm sure you'll lvoe them
Whatever you do don't get 2 males together in the tank. In my experience male sparkling gouramis are rather territorial. Especially if one's already established. Unfortunately I haven't been able to find any females at all in my local area.