In Cathedral Level 3, I have the chance to either fight with or without the Templar. I thought the enemies would grow stronger relative to the size of my party, but typically this is announced by the game and there was no such message when I accepted him straight away.
Will I get experience and drops from his kills/last blows/etc.? Will I be getting less of it overall? perhaps more?
Only when you connect with a real-life player will the monsters get harder.
The in-game NPCs that you can recruit make no difference to the difficulty and you will continue to receIve the same experience and loot.
On another note, the loot differs from player to player, so you dont have to worry about people stealing it.