If you have legs and can use them 90 minutes is plenty at TXL. I don't know if there is a shuttle because I never looked for it, I only know you wouldn't need it. One can walk from any point in TXL to any other point in under 20 minutes. Immigration is not really an issue in Germany and not comparable to the rigamarole at U.S. ports of entry. I've never heard any complaints about lines or extensive questioning at TXL.