I think you mean 720p. You can mix different formats or you can convert everything to whatever format your final output would be. Lot’s of different ways to do either, and lots of different reasons to choose certain workflows. There are way too many variables to tell you what the best option is. The simplest answer is also the most time consuming and most expensive (buy a Kona 3 and digitize everything in one format, the format that your final output will be).
Honestly, you may be a little out of your element. No offense, and I mean that sincerely. So proceed with caution. I suggest reading the FCP manual as it explains general HD video concepts very well.
Sean