Let's say I had a small map with just one CP

depends the layout 100%

Well you don't want your map ending really quickly, but then you wouldn't want it lasting for ever. Nobody really knows with just 1 cp unless you playtest it. So get it into the game weekend.

This almost reminds me of that one gimmick class challenge map where the teams would finish the course and fight over the cap point. Only minus the gimmick, of course. I'm leery of single cap points simply because the objective never moves, making the CP area something of a deathmatch.

Just an idea, but have you considered adapting the dom_ gametype percentage system setup at all? It might be more king of the hill styled to track progress that way rather than whoever caps first, and alleviate that mad rush in the beginning of a round to get there before the other team.