I don't know about DAL but does JB still have the >20,000 lb requirement on it's Part 121 TPIC minimums?

Have I seen people hired at Delta without having flow > 20,000 lb aircraft... yes. 1 for sure.

How long until you can be PIC on the Brasilia's?

FWIW, a friend of mine had a Cape Air pilot on his plane. That pilot's plan was to skip the regionals and go straight to the majors from C402s. I don't know how possible that is, I guess it'd depend on who is hiring.