Don't do that. FCP does not work with either of these frameworks well. Handbrake is not suitable for this.
I've not tested it with DV AVIs, but the simplest thing may be to optimize the media. Not sure if FCP will do that. The other option is to convert to QuickTime. You can use MPEG Streamclip to do this.
Depends on what kind of quality you need after you get around to editing them. The more digital information you retain beofre you finish and compress, the better the result. You can always delete the larger files after you complete your project. And I would suggest MPEG STreamclip…much more control.