Well i'm doing the same, 4 shammies and a prot pally.

It wont work like that (as prot in arenas or whatever) but there is little difference in levelling a group of 5 than a group of 4 imo. My pally is on a different server, and way ahead in terms of levels atm, 61 vs 43, but I tried out prot over the last few days and I think it'll make instances breeze. Warriors are all well and good as tanks, but keeping multiple mobs stuck to you as a prot pally is soo easy.

If you want to do any PvE at all at 70, take the pally along for the ride. If not, don't bother would be my advice, seeing as you have one at 70 already.

As for levelling, boosting to 40 then a mix of mainly quests and a few boosts post 40 is working for me. There are loads of posts about which is faster, and there's no definitive conclusion yet imo.

Good Luck

Ky