I'm thinking you are going to need to add some soil.* You could plant anything that will stand the sun conditions, but something with shallow roots will probably be good.* Carex sounds good to me.* How about one of the low growing sedums?* They will cascade over the stump.
As to the pH, if it was an oak or conifer, expect the pH to be on the acid side.* It really shouldn't matter as most plants prefer a pH from neutral to slightly acid.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.