I like 10- 12 week old fryers. Anything older is better for roasting or crock pot. Rule of thumb used to be 5lbs by 10 weeks. Any older does add more kidney fat and lymph node fat under the armpits. I feel it changes the flavor too. If it gets older and you want to fry it, I soak portions in milk a while or even overnight. This is just my opinion, but been processing now for abt 32 years.
Depends on the weight. 4-6 lbs is best for standard meat breeds. However if this is a small breed you will have to butcher at around 2 lbs. You did not state the breed.
Do not think that you need to wait for rabbits to be adults. Like any other meat animal you want to eat them while they are tender - this means young. If you are holding them until they are adult rabbits, they will be roasters and stewers, not fryers. Definitely butcher them now.