I wouldn't put any more fish in there. You are fully stocked and you should remember that the cories you have all grow to a decent 2-3" and are very active fish. Realy, cories like these should be in at least a 15 gallon. The guppies would be fine with gouramies BTW - it is bettas that have problems with male guppies because of their similar appearance
Also, I wouldn't put a pearl in there. Pearls do quite a bit of swimming and, as they get to 5", would be seriously over-stocking your tank. Opalines (three-spots) are not a good choice for a different reason - and that's their aggression. Especialy considering they get to 6" and are very territorial, they would do terribly in an over-crowded tank with peaceful fish. A single dwarf would be possible as adding a 2" fish to the bio-load in your tank will not be over-stocking your tank TOO much. However, dwarfs can be quite sensitive so putting them in a stressful, over-crowded, environment is asking for trouble. As for sparklers, these are tiny but do better in small groups (say a trio) in a heavily planted tank. You may have room for one, but not for three so I wouldn't get sparklers either. What you could attempt is a single male honey gourami. These are hardier than dwarfs but don't exceed 1.5". They are otherwise very similar and males develop lovely coloring when in breeding condition. Don't confuse them with colisa lalia (the dwarf). Honeys are colisa chuna/sota. With females, I always feel it's better to keep them in a group but the males, at least, being territorial fish, do fine singly, so I would stick to picking out a male (brighter, pointed/longer dorsal, may display its territoriality if you observe them for long enough in the LFS). You are still over the limmits when it comes to bio-load though (and physical space for the cories) and I personaly wouldn't reccomend adding anything more at all.
I should add that I have no idea how you can have 0 nitrAtes with your tank fully stocked and not realy that heavily planted. If you are using a test that uses those strip things, you may want to get a new one
Better yet, get a liquid-based one. Otherwise, I guess you have some super-fast growing plants or do huge water changes every day
If, in fact, BTW, this is the case, you could get a trio of sparklers or a single dwarf (prefferably male) gourami as you evidently are not, as far as bio-load is concerned, over-stocked (however, the cories still need more space). Just check these realy are your stats first though (I suspect they are not).