Nothing, they will probably become friends. Female turtles are very compatible. Now, you don't want to have multiple males in a tank with out more than enough females to go around because they will fight.
No, a mussel or clam cannot be put in the same tank as a baby turtle. Baby turtles, as well as adult turtles will eat mussels and clams.
Depending on the species, one or two. Turtles will keep growing through their entire lives, overcrowding or expecting them to only grow to the size of the tank is something that usually leads to dead turtles.
yes
No. Not unless you're prepared to take care of all the baby turtles they will have together.
Yes you can! In fact it is recommended that you do!
I have mine together in the same tank and they are fine so I don't see why not.
If it's a huge tank, has an outstanding filter, and has two or three large basking spots each with UVB light (large means that it can fit all three turtles on it).
No they will kill each other
Yes they can as long as you follow the rule about 10 gallons per inch of shell for all turtles. For instance= 2 2 inch turtles need 40 gallons not just 20. Make sure the turtles are near the same size and you don't have two males or there will be aggression. Also, if you have a male and a female prepare for them to mate. Also, the tank gets dirtier a lot faster.
About a 20 gallon tank.
tutles and fish should NEVER be kept together
probably not. they have different kind of food and they might get mixed up