1. I think that you can use any card that you're drawn to for a significator. If it feels right, then there's nothing to stop you from using any card in the whole deck. Some people might disagree, and only use a card which matches their sign, looks or personality and that's fine too.
Generally speaking, nothing is an unbreakable rule in tarot. There are as many different reading techniques as there are tarot readers.
2. Once again, it's largely a matter of personal preference. If I am using a significator, I usually pick it first and then focus on it while shuffling, but sometimes I'll leave it in the deck.... it all depends on what I want to do at the time.
Basically, my advice is do what feels right for you, you can't go wrong