As far as switching from live to f/t, some snakes will switch over with no problems, some may take a little time and patience to get converted, and there are some that will not take f/t, no matter what. It's impossible to tell what your snakes will do without trying...

As for feeder weight, I follow the 10-15% rule. I feed my growing girl 10-15% of her body weight weekly... So, she's around 500g now, she eats two 25g mice a week, and I will probably be switching her to small rats very soon.