Hi,
I'll try and answer your questions one by one - I'm no expert but have being doing a lot of reading on dwarf cichlids recently
1) the most common dwarf cichlids you will come across are Rams - these could easily be kept in a 30 gallon but require very specific water requirements that are hard to meet (warmer water than most tropical fish are comfortable with and very low nitrates). However, Bolivian rams are much hardier and have great personality (although are not as colourful as rams)
Another common dwarf cichlids are apistogramma (there are many species) which are often kept in pairs or 1m2f trios - they are very peaceful to most fish (don't mix more than one species of apistogramma) except when breeding where they will become territorial - although most fish will still be fine with breeding apistogramma.
The other common dwarf cichlid is Kribs - these are African cichlids unlike the South Americans mentioned above - and while they are peaceful for African cichlids they are much less community friendly than the above.
There are other dwarf cichlids but these are the most common.
2) I think I covered that in the above
3) any dwarf cichlid will be fine in a 30 gallon in either a pair or 1m2f trio but I would only keep 1 species to keep it safe
4) they are usually kept with dither fish - these are small shoaling fish whose presence in the water give the dwarfs confidence that there are no predators about and it's safe to come out. Without these apistogramma in particular can be very shy. For dithers I would recommend small tetras (lemon, cardinal, neon, red phantom tetras, etc) or pencilfish amongst others. Anything that keeps to the middle and upper levels will be fine as the dwarf cichlids will stay in the bottom third. Peaceful bottom dwellers such as corys should be fine but you should be careful of the dwarfs are spawning as they will get territorial and sometimes corys don't know when to stay away.
Hope that helps
