I'm responding solely on the fish species issue.
In a 30 gallon tank, a group of 10-12 Serpae Tetras would be it for fish. This sized group in this sized tank is minimum for this species. They are notorious fin nippers but this group will usually curtail this, but there is no room for other fish (maybe some substrate fish, but no upper).
Black skirt/widow tetras also have a penchant for nipping, so they need a group of 7-8. You could have another upper species in a group, if carefully chosen. Or you could increase the skirts to 12 and that would be it, again with some substrate fish OK.
General comments on the shoaling/schooling issue since you asked. No freshwater species is truly schooling in the sense that we use this term for certain marine species that hunt together. Shoaling is a better descriptive term, and it means the fish naturally live in largish groups and they must have a group as this need is programmed into their DNA. Numbers vary, but more is always better than minimum numbers. The fish will be stressed without adequate numbers, at the very least, which means a weakened immune system leading to further issues as time goes on. Some shoaling species have other needs for the group, such as the hierarchial fin nipping issue mentioned above. All characins (tetras, pencilfish, hatchetfish) are shoaling species, as are the rasboras, danios, barbs, cory catfish, loaches, rainbowfish.