I agree with Deda, raw is best, but is hard to change kitties eating habits, second best is high quality canned.
As for the sucking, it is a calming action that cats who have been taken from their mommas at too early of an age continue to do. My Snowball did this until he was about 5 years old, he was barely 6 wks old when we got him from the HS.
Cat maid to:
Puddles, female Himalayan (16 yrs)
Jasper, male Ragdoll ?? (approx 9 yrs)
Sweet Pea, female silver tabby DSH (approx 7 yrs)
Rose semi feral, a cpietra rescue, female tabby (approx 6 yrs)
Snowball RIP (1991-2005)
In a cat's eye, all things belong to cats.-English Proverb
“While we are free to choose our actions, we are not free to choose the consequences of our actions.” Stephen R. Covey