I couldn't find which post that talked about switching male rats weekly to make them more interest in mating (don't know for the male of the females).

I just tried that and the next day one of the tubs has pinks. My concern now is would the male harm the pinks since they are not his?

I don't use nursing tubs for the females because I'm not very keen on catching and holding them to see if they are prego, because I've been biten twice, still has some psychological effect.
And I can't tell from just looking at them from the top.