need some questions answered about mud/musk turtles

Good evening folks. I just received a 40 gallon breader tank am setting it up for some future turtles but have a few questions I hope you guys can help me with. I know this is long but want to make the right purchase the first time!

I am considering:
mud : mississippi, three striped, eastern, yellow
Musk: stinkpot/common

I am looking to get three hatchings or yearlings. I want something the children can watch grow over the years coming to my room. In my research I have concerns:

1. Is it safe to keep a couple to three of these guys in the same tank?
2. Is there more issues with males since the gender can't be determined for this young of turtles. should I worry about this?
3. How much more delicate are hatchlings compared to yearlings? i don't mean physically (i.e. handling), more in caring for - risk of drowning, etc
4. would you recommend one type and age over the others?
5. Is there anything else I need to consider?
6. i have found a list of places to purchase but have no idea over the reputation. The Trader Rating here doesn't really cover or give much detail on these places. Is there a way to determine who I should avoid?

my background:
I have kept sub-adult RES WC and chameleons., so I have worked with critters in the past but never mud/musk. I had a bad experience purchasing a chameleon at a trade show and am trying to avoid that for the turtle, the children, and myself.

Just found this thread--sorry I missed it earlier!

I am a turtle newb (only a couple of years in the hobby), but I do have a common stinkpot. I bought her(?) as an adult, and she is currently sharing a 20 long with two yearling mississippi maps. They will be moving to a 40 breeder here at the end of the week. Ultimately the maps will need a larger tank in a couple of years, and then my stinkpot will have the 40 to herself.

I bought the maps from The Turtle Source (located in Florida, all captive bred animals as far as I can tell) and have been extremely happy with their service and customer support. They are running a 10% off sale right now and have a number of stinkpots for sale, ranging from started hatchlings to yearlings and adults. I'd recommend starting with a yearling or older just to make life easier on yourself.

If you are still looking for turtles, you also might want to consider giving them a call and ask you questions--I do know that they say that mud turtle hatchlings can be challenging because they are so small and thus are relatively delicate. Austin's Turtle Page has a great set of care sheets and FAQs that I've found extremely useful as well, so it might be worthwhile to check it out.

Hope this helps!

Catherine