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.
Followers do not inherently affect monster HP/difficulty, experience (XP) share, or item/gold drops. They're designed to make single-player easier, and there is no statistical disadvantage for using them. They contribute 20% of their added Magic Find, Gold Find, and XP bonuses.1 2 3 4 And, like D2 hirelings, their kill XP counts as your XP.
Also remember that followers cannot be used with parties and that D3's party size only affects monster HP. There is no party XP, item, or gold split—everyone gets the same XP, and drops are private (so as to reduce infighting).
Everything stays the same. He is only a Follower/Hireling who does not affect the game except by helping you and giving you his equipped item stats, e.g. magic find, etc.
You don't get less experience, but you will lose any gold or magic find percentage.
