"are all Doms polyamorous, moreso than even vanilla men? does this way of life attract men who are already polyamorous?"

I can't speak for men but not all Domme's are poly either. I'm certainly not. I don't share well with others.

I think there are many attracted to the dominate way of life because they figure it is an easy way to get their pleasure without having to worry about their partners. In my not-so-humble opinion, that mind set is for psuedo-Dom's.

In your negotiations be clear what you need and ask the questions about their partners and commitment to monogamy. If you aren't getting the answers you want, there are plenty of other fish in the sea.