Switching between iPhone and non iPhone

Two questions (one for me, one for a friend). I've searched but can't find definite answers.

1. When I get an iPhone, I'd like to be able to switch and carry my Crappy Old Phone (COP) sometimes (like, if I'm going to have to leave it somewhere not entirely secure, or if it's going to get wet). Can I keep my current COP and just put the iPhone SIM in it? I understand I won't be able to use the data plan, but I just want to know if I'll be able to make and receive calls on the COP.

2. A friend is going to Canada and apparently found out it will be crazy expensive to even turn on his iPhone there, since he'll be charged for each pushed e-mail. Can the push feature be temporarily disabled? Can he put his SIM card in his old (non-web-enabled) phone, which would prevent him from receiving these pushes, and still use his phone?