All sound good to me. If we go with infantry only, we could rule out an upgrade to MI - just another limitation. In this scenario, I would also rule out Persia and Rome, as their UUs are powerful enough to be near equals to knights. Even then, the limitation is a major factor only in the middle ages, as the game should be in hand by the time tanks appear. What would be tough is making domination the only victory option.
The war scenario sounds wild. Presumably we declare war with anyone who even establishes communications with us, and never make peace for the rest of the game. This sets one's course directly for a domination victory, in which case it doesn't sound so tough at all. Now if we have to win the space race...
The 5CC would make for a real challenge, but would be a great learning experience in how to maximize production and trades. I would suggest not allowing a diplomatic victory in this scenario, as all you'd need to do is survive, stay reasonably close in tech, and prebuild the UN... given the small size, popularity wouldn't be a problem.
The first two also sound like good options for a SG.