Rieklings will seemingly equip any armor (except shields) as well as use any greatsword you give them. You can also use them as a tiny pack mule. Be mindful though, it seems every time I've entered a black book and returned, most of the stuff I've given to the little goblin has vanished. He still had a few things, but about 90% of the gear and misc I'd given him was gone. I was especially sad to find my 20 copies of the Lusty Argonian Maid missing from his pack. I had a shelf back at my house all ready for those books. Sadface. Also O-face.
It's not just Apocrypha that causes it. Sadly, the Riekling follower just can't hold onto much in the long term. He lost his custom enchanted Orcish Greatsword(his "Pokey") and all the armor he was carrying. In fact, the only thing he retained was 10 torches he was muling around.
And it just seems to happen randomly, whether or not you visit Apocrypha.