There was a thread recently concerning this very topic. It mentioned dremels, rotozips and hole saws. I thought I had subscribed, but I can't find it. Maybe in the DIY section.
I used a flat head screwdriver with a hammer, lightly chiseled perferations, and then broke it along the perferations. a couple didn't break quite the way I wanted, but most did (pots are cheap, so buy more than you need in case of breakage. cheaper than buying powertools you don't already have. I have an angle grinder now, so I use that.
Yes pots should be fine, clay ,Terracotta, is actually largely inert when fired, why do you think they always find broken pots and vessels on the sea floor that are thousands of years old?
I think the only thing about it is that it tends to really be an eyesore but if it's a breeding tank or you don't mind, go for it.