BOosting involves 1 high level running 2 linked lower levels through instances... make sure you dont link your high level accounts with your low level accounts. If you just need 2 more chars you should probably get two accounts and link them. Then have one (and only one) of the high levels run instances after youre done with the lowbie stuff. SM most people say will take you from 20-40ish, thats a big chunk right.

For classes id look at roles and synergy. You are going to make a pally to tank.. you have 2 priests and a shaman as either dps or healing, not sure. Assuming those 3 will be dpsing, then yeah a resto shaman would be beneficial. 2x totems and good heals, off dps when you dont need to actively heal. Synergy is all about.. well you wouldnt drop a rogue in that group. The buffs you want for your group are going to be all casters, so a rogue would not be as beneficial as another caster, thats all I mean by synergy. Just throwing an example out there.