As to the species, it really depends on your supplier...octos are a "hit n miss" sort of animal to aquire, and you have to roll with some punches. Unless you can find a specific animal you want already at a store, you will most likely have to go with a bimac, but they don't really qualify as small...but they do start that way! O. digueti comes in now and again, but with Mexico's new trade restrictions, I wouldn't bet on it...
Best bet? Probably start with a bimac...and see where he/she leads you! Happy Cephing!