Questions about NPCS, purchased mounts, and mercenaries

[Falontani]

In a campaign I am running one of the players has purchased an animal with an advancement (they purchased a trained hippo), has hired a commoner, and a mercenary. The group is still relatively low level and there aren't many players in this campaign so the NPCs are contributing.

Would it be reasonable to track the EXP of the Commoner and the Warrior and level them as they progress? (The party is stuck in a dungeon crawl that neither the party nor the NPCs were expecting, so they can't leave)

Would it be reasonable to track the EXP of the Hippopotamus? Would the Hippo count as an 8th level party member for xp purposes? (that would truly mess with the party's xp)

I am asking before I give the party the option, and depending on the party's response will be whether I track xp for them or not, but I'm coming to you guys to see what your opinions on it are.

Finally: the NPCs both have NPC stats: the commoner has 11 11 11 10 10 10 while the warrior has 13 12 11 10 9 8, would you have them grow more powerful if they gained xp (say by giving them the 15 14 13 12 10 8 spread) and how would you handle their class(es)?

[heavyfuel]

The Commoner and the Mercenary? Sure. Track their XP. Add them to the party level and while this will mean less XP for the players, it will also mean more survivability. Also, I suggest you don't mention this to players to avoid them meta gaming. Just distribute their XP as usual, and then say they notice the Cmmoner and Mercenary fighting a little better after they level up (the commoner might even get a level in Warrior)

The hippo can't gain experience on the fact that it's an Animal with 2 Int (using Pathfinder stats here)

I wouldn't give them better ability scores, no.