The Celts weren't just one group of people. Celtic refers to a whole ethnolinguistic group consisting of tribes across Northwest Europe who spoke Celtic languages and a Celtic culture. It doesn't refer to one group of specific people exactly. The Picts also weren't just one people, they were a large group of many kins or tribes. It's believed they probably were originally Scandinavian people and it's not an uncommon belief that they were ethonliguistically a part of the group considered Celtic. Some authors who have written on the history and language of the Picts are: Katherine Forsyth, Sally M. Foster, Dauvit Broun, and W.A. Cummins.
As for magic from them, you're probably going to find very little if anything at all as those Pictish kin were mostly of oral tradition other than the relics and carvings they left behind.