HD footage in SD project

[Kristijan Stramic]

Hello!

I decided to make a 20 minute movie about skateboarding and it's lifestyle. I have one SD camcoder Sony DCR-VX2100 and AVCHD camcoder Sony HXR-NX5. I will be using VX2100 with Century Optics fisheye 0.3 adapter.
I set VX2100 to 16:9 aspect ratio and NX5 to 720/50p.
For video editing I use Adobe Premiere CS4.

1. Should I make SD 16:9 project or HDV 720/25p project?

2. Let's say I make SD 16:9 project and then import all of the VX2100 footage. Can I also import HD footage from NX5, and start editing SD and HD together, or do I need to convert it to any other format first?

3. I really don't want to ruin the quality of my HD footage, so what's the best format to export to if I will be working on a SD project?


Thanks,
Kristijan