As far as I know most pellets are Timothy Hay-based. Mine eat Timothy-based just fine but maybe alfalfa-based is what yours ate previously. You could try mixing alfalfa and Timothy pellets together as I have done that before and gradually switched it to all Timothy.
I usually feed all my buns a alfalfa and Timothy hay mix of about 3/4 alfalfa and 1/4 Timothy and it keeps them very healthy and they like to eat lots (especially my Lop).
As far as pellet amount I offer 1 cup per day and my buns aren't overweight. Then again they don't eat all of it every day so they're probably really eating 3/4 cup daily. I offer babies unlimited pellet as they are growing.
Vegetables and fruits wise there are a few rules of thumb that I go by that keep mine happy and healthy. Leafy greens (romaine lettuce (stay away from iceberg), kale, radish tops, carrot tops) I give in quantities if half a packed cup (I've heard you can give up to a full packed cup) daily. Regular veggies (carrots, celery) I give 1 tablespoon of daily. And fruit (bananas, apples, coconut, pineapple) 1 teaspoon every day. Herbs (parsley, mint, rosemary) you can give in unlimited amounts and it shouldn't bother them.