Are you editing it, or do you need to deliver raw footage?
If you're editing it, I'd just edit natively 16:9 at 30P. Then once you have a picture lock, I'd copy all the clips to a new 4:3 sequence. FCP will probably letter-box your footage so go into "motion tab" and set the scale back to 100%.
Then copy that clip. Select the rest of your clips. Paste attributes, making sure the motion box is checked. Now all your clips should be in 4:3. If you cropped off something you need, then you can go on a shot-by-shot basis, using the same motion tab, and re-align your footage.
As for 30P and 60i,.. I dunno,.. I'd just leave it at 30p. It'll end up being 60i, but with duplicate fields,... I think.