You can use Google Voice with a non-smart phone, but it will be somewhat limited. If the only device you carry is the dumb phone, you won't be able to do any calling over wifi (obviously).
However, if you carry a wifi android device with you, you can set up Google Voice over wifi using a few methods, such as GrooveIP, or SIP Droid with these setup instructions.
To only show your Google Voice number to callees when using your prepaid, follow Option 1 in this answer. Note that it will cost you 10-15 seconds each time you call someone, as the phone will manually dial the number after calling your own google voice number. Receiving calls won't take up any extra time.
If you don't care about your friends and families having 2 numbers for you, you don't need to do anything different for your prepaid. Just know that if they call you on your non-GV number, you won't be able to accept it on your wifi android device (it won't ring). You would need to explain that they should only ever call you on your GV number.