Just so you know, the synodontis will grow up to 6" or so. It will stay peaceful, but you'll probably need a bigger home for him down the road or be able to return him to a fish store when he outgrows the 10g.
You could probably add a school of small tetras (cardinals, neons, lemons, embers, diamonds, etc.), or else a group of male guppies (you don't want females in a planted tank, the babies will be able to hide to well and will overpopulate you very quickly). Maybe 4-5 of whichever you choose, as long as you plan to move the synodontis when he's too big.