I have been using fast flight strings with Rose Oak limbs on vintage Bear risers for many years without any problems.
Feel free to use fast flight with the Rose Oak limbs, and B50 with the vintage limbs. You won't be hurting the riser in any way. You will be impressed at the way the fast flight and Rise Oak combination performs. You will need a higher spine arrow with the Risd Oak limbs. For me, all other things being equal, the Rose Osk limbs need a higher spine group in carbon shafts than the vintage Bear limbs.
Bear TDs are great bows. Have fun with it.