That's a pellet rifle, not an airsoft gun. It is not a toy, it will kill small animals.
Anyway, that modified MP5 is fine. It's compact and lightweight but has a sliding stock. The G36c would be a bit larger than that, so unless he really wants a larger weapon, that one is fine.
I don't really mind size (except when it's as tall as me). A Masada would make my day too.
For the Echo 1 MK5K, I'd probably add a small foregrip plus a Holo sight, probably add a small laser to the right side.