Additions to a 10g

Why dont you get a few more neons, they like to be in groups.

Angelfish will eat neons. In fact, I believe they are natural food for angelfish in the wild.

A 10 gallon is too small for a fully grown angelfish. I think the smallest recommended tank for them is a 20 gallon tall since they grow pretty tall. You either have a larger tank you're planning to move the pleco to or will be setting one up, so you can just plan to move the angelfish with him to the larger tank maybe?

I'd get a group of 3-5 cories for that tank, then add more fish once you get the angelfish and pleco moved. Maybe a betta, dwarf gourami, paradise fish or even another school of tetras once they won't be lunch

Well, neons are an part of angel fishs natural diet. But, in a 10g you can't keep an angel too long (it'll get too big) so, unless you've got a large angel in there i don't think it ate you neon. Or maybe it did if you have a not so small angel and tiny noens.

I'm just a newb, so I'm not sure about the angel eating your neon, but if you never found him, I would say it's a good possibility. I just suggested the neons because they're small. Maybe someone else could give you better advice. What do you have in your tank, ei: rocks, plants, etc.. You might need something for fish to hide in.