1) can. But the space like u said might not fit a skimmer especially for a big one
2) up to you. Depends on the fish. Big fish, you need more swimming space. There isn't a rule for minimum
3) depends on your budget, you can check out deltec, skimz , that's my preference
4) not a must. Depending on the normal temp of the water without chiller. Mine temp is about 30-31
5) I would give a proper wash and start dosing bacteria and do the proper cycle
6) not a need, depends if u like the look. Without sand, easier to maintain