Cories are omnivorous so do best on catfish pellets or sinking live/frozen foods. They are great, active fish and good scavengers of missed, uneaten food. They should be kept in a group of at least 4. A nice variety, and a personal favourite, are the blackfin corydoras (corydoras leukomelas) though others are nice too.
Otos are known to be fragile and are strictly herbivorous. You need to provide them with algae waffers and they will only thrive in the presence of live algae. They should be kept in a group of at least 3 or at the very least in a pair as they are social fish.
When it comes to gouramies, don't go for dwarfs (colisa lalia) as they have become increasingly in-bred and fragile recently. If you are looking for a small top-dwelling gourami, go for a trio of honeys (colisa chuna/sota - a male and 2 females). These are nice and small, peaceful gouramies and the male will develop a wonderful blue-black throat and underside to contrast with a bright orange background once he is in good condition. The females are also attractive with pale yellow and delicate shades of grey and brown.
Another possible gourami option would be a single thick-lipped gourami. These are larger but even more beautiful and still generaly peaceful. Unfortunately, you wouldn't have room for more than one but they are well worth it for their striking colors. They grow to just over 3 inches and (because you'll need it considering the number of confusing names gouramies are given...) the scientific name is colisa labiosa.
Concerning livebearers - I'd stick to guppies or add a few platies but don't mix genders as you will end up over-stocked (unless you are prepaired to remove fry as they grow up).
As for your stocking, you currently say you have 1 male guppy and 6 tetras. Be aware that the tetras especialy won't appreciate a sudden change in pH (especialy and increase) so make sure you introduce them gradualy or, if I'm too late, keep an eye on them and don't add any new fish for at least another week or 2.
Using the 'inch per gallon guuideline', you don't have space for so many more fish as each tetra (assuming neon) would count for about an inch and the guppy would be another - that's 7 inches already. That leaves you three inches of fish and that isn't a whole lot. Your 5 gallon was over-stocked according to the guideline. Obviously there are exceptions but you should be aware of this and test your nitrAtes to make sure you can keep them below 40 without more than one partial water change every week. This is the best indicator of stocking levels. And, of course, also watch your fish for signs of distress or illness and reduce your stalking levels accordingly.