I recommend to feed the same prey as previously fed for at least 3 to 5 meals, once well established in your care if you want to switch you can.
As for what to feed him it is up to you, what work best for your animal and is convenient for you. Gram per gram of prey it's all the same the advantage with rats is that once your BP is an adult you can feed a single prey versus multiple.
in your opinion will it be difficult to get him on f/t or prekill rat if he eats live mouse until he gets to the point of eating two..