Well, I live in NTSC world, but I use the Cannon XF100, and I like to shoot at 720 60p for anything with much movement.
In NTSC, Blu Ray supports either 720 60p & 24p, or 1080 60i & 24p. So one has to choose between 1080@60i or 720@60p.
1) Please note that I favored interlaced video because most of the footage will have fast movement (2 Jiimy Jib and 2 Steady Cam) and I think interlaced video would be better in that case.
2) Also, I have read somewhere that there is a way to edit using preview-quality copy of the footage and then exporting using the original HighRes footage. is that possible? I am not sure if my system would handle a 9camera multican sync of 1080 footage.
3) Also, do you think 1080 50i is too much? I need to export to BluRay eventually but I really do not need SUPER high quality. would 720 50i do?
1) I'd choose 720 60p over 1080 60i, and you don't have motion issues with 60p. Interlacing can have its drawbacks.
I tend to shoot most stuff at 720 60p, except for green screen shots and interview talking heads shot at 1080(30p). The reason I shoot those at 1080 is to give me resizing options in a 720 editing sequence. I can mask a jump cut by changing the size of the talking head shot, and the same basic concept with the greenscreen shots. I choose 30p over 60i 1080 because 30p has better low light for indoor interviews and there is usually little motion; and you don't want interlaced footage for green screen.
2) NO, it may be possible to edit proxies for these formats, but why would you? Using Premiere, the Cannon XF codec and Sony Xdcam codecs cut like butter in their native forms. The are also speedy to export when finished. If I use two comparable sequences, one Cannon XF, the other AVCHD, the Cannon fooage will export about 3x faster.
The GoPro fotage is the only one that you could get some benefit transcoding it to intermediate before editing. And in that case I would covert the footage to MXF OP1a in Adobe media encoder. However you may find that the native H264 from the Gopro work just dandy for you.
3) It really depends on what you want in the end. Again, my needs are usually meet at 720. The consideration is the Blu ray spec, if that is what you are delivering to.
Google Blu ray specs, then shoot so as to not sabotage those specs. For example, I think that 720 30p and 1080 60p are not supported by NTSC Blu Ray.You'll have to check out the PAL specs.