There are a lot of opinions out there for treating diabetic cats. I'll tell you what I did with my sample size of one (Mobius.)

She was isolated and closely monitored for several months until she was regulated. (I would probably let a cat out sooner, based on what I've learned.)

I let her into the general population; she gets wet food twice a day and free feeds dry. This is contrary to the newer protocols, but Moby is 14 and I don't want to disrupt her schedule too much. In a year and a half her fructosamine, which measures long-term control, has dropped from 480 (fair regulation) to 421 (good regulation) and her weight has gone from 14 to 11.8 lbs in the same time (she was overweight at 14 lbs.) She has also dropped from 3 units 2x/day to 2 units 2x/day.

Every cat is different. If you can do blood testing, and feed mainly wet food, and "tune" his insulin to his BG reading, you can manage a diabetic cat which lives in the general population.