can i switch 3g networks ( from bell to rogers )

Probably not but you'd need to confirm that with Apple or Bell

For the most part, yes - the provider you use is determined by the SIM card in your iPad, so at its simplest level you just need a Rogers SIM and away you go.

The main thing to be aware of is service contracts - make sure you can cancel Bell service without a penalty or you'll find that you're still paying Bell for service even though you're not using it.

The iPad is not sold as carrier locked.

If Rogers has a data plan only for the iPad and other similar type devices and they certainly have a micro-SIM card for the iPhone 4, contact Rogers and ask.