If you want to have a true mixed species tank, be sure to start it with plenty of guppies, catfish, and plecos. The catfish and plecos will help to clean up after the turtles, and the guppies should be a self-sustaining food source. As far as food... variety, variety, variety! Quite frankly, there aren't that many turtle diets, so supplement with fish food: frozen; freeze-dried; larger pellets (cichlid & pond).
If you start with babies, you might have to tone down filter strength, but increase in strength as they grow. I know all the literature says they like quiet bodies of water, but you have to balance the concerns of natural conditions with practicality. I strongly recommend an undergravel filter with several powerheads for supplemental circulation. Spend the extra money for large aquarium pebbles. Generic gravel from Lowe's or something similar will most likely lead to headaches with excess algae growth.
Any literatue written by David Kirkpatrick is excellent. Not only is he an expert in turtle care, but his own personal interest is in the mud/musk group. He has a book published by t.h.f., and several of his articles are posted on the web.
I wouldn't worry about multiple specimens of mixed species as long as you don't crowd them. There are a few breeders, but most are farmed and not usually available as hatchlings except in August and September. The turtles are very hardy, so unless you are looking for something very particular, I wouldn't worry too much about the reputation of the supplier.
My own personal recommendation is the Loggerhead Musk. It will probably cost 2-4 times as much as the other species you mentioned, but it is a magnificent beast! I liken it to a military tank with the head of a monitor lizard... needless to say, I have one and I really enjoy it. It is in 20L turtle tank, but I have never seen it bask. Though it is the only turtle in the tank, it has plenty of fish company.

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