As far as keeping them with hardshelled turtles DON'T. Some people do but the do have specific requirements and because of future aggression issue coupled with this it is best not to combine them with hardshell turtles.

I don't think a turtle dock will do any damage to them.

As far as size of tank goes you can start a couple of hatchlings in a ten gallon tank but they will soon out grow it so you are advised to get something a bit larger to start with (20 gallons or larger). Most softies get fairly large (especially the females). Chinese softshells get to about 14 inches in size. This is large but compared to a Florida softie female it is a small animal.

Spiny softshells stay on the smaller side too (males anyway).

There is one smaller specie (forgive any mispelling) Dognia Subplana (or something like that). But it is NOT a beginner soft shell and also not that easy to find.

Get a Spiny or a Chinese softshell and hope you get lucky and end up with a male.

Oh as far as keeping more than one softie together. That's a crapshoot. They can get very aggressive injure eachother.

Check out the kingsnake softshell turtle site. They may have some answers for you there.