If you have a 25, it stops taking in XP, so the other pets that participate in the battle will absorb the extra XP. The fastest way then would be to use two 25s to boost a lower level pet through the tamers (once you finish each part of the chain they become dailies... hope I got that right. At some point they do become dailies.)
If you do 1 beast, 1 mechanical, then my suggestion is either a flying (strong v aqua) or magic (strong v flying) for the third spot. That should give you pretty good coverage for wild pet battles and some versatility in tamers up to a certain point. Or if you plan on doing a lot of wild pet taming, then get a Terrible Turnip for third spot, it will help a lot in catching wild pets.
Yes, pet battles are a great way to level your toons as well. I didn't think I was going to bother with some of my alts but they got so much XP from pet battling it seemed a waste to not take them all the way to 90.