i actually think it is also a matter of timng
shrimp are very teritorial by nature, although they dont seem to be able to protect/defend thier homes too well, they are more successful against other shrimp, especially smaller ones.
so, if you have a larger shrimp, especially the cbs or the skunk cleaner, then adding a smaller one, at a later date, will probably be a lot of 'action', as teh bigger one keeps chasing the smaller one...
if the smaller one survives, it will be cool as they will prolly pair/mate, (same species)... my pair were constantly carrying eggs... i was lucky enough to see a hatching once... it was friggin amazing to see a bazillion eggs hatch in about 3 seconds...
(btw, they carry the egg around thier back legs, tucked under thier tale... you can see them up close with a flashlight, and as they get bigger the shrimp are constantly groomin and flexing thier legs to fan out the eggs)