Are you sure about the sexes for all of them? If so, I assume they're more than 4inches long each? If that's the case, 55gal is not nearly enough room. Think 10gal of space for every inch of turtle.
1. Go ahead, if there's room. I have a few plants that suction to the side of the tank (made for dry terrariums but they hold up well in water). My res like to sleep on them and they pretty-up the tank a bit.
2. Ideally, the basking area should have enough room for all turts at the same time so do whatever you have to to make it bigger.
3. Anything smaller than their heads can/will be swallowed. That's not a risk I'm willing to take. Take out the rocks/gravel you have in there and replace them with large river rocks. I get mine for $1 a bag at the dollar store. You don't need to cover the bottom of the tank because that just makes cleaning more difficult. I have about 10 large rocks in mine and they like to push them around.
4. There is a list on the "Feeding-Nutrition" board up top, one is "What to Feed your res" and the other is "What NOT to feed"...it pretty much covers everything
And welcome

The things that come to those who wait may be the things that were left by those who got there first - Steven Tyler