I don't think adult food will cause any problems, esp if he's still having some kitten dry. Kitten food has a slightly different nutrient profile - more calories, more protein ? and fat than adult, but depending on the brand there sometimes isn't very much difference. I always go by the fact that animals in the wild don't have special kitten food and grow up just fine. A kitten really just needs some extra calories for growth (prior to neutering especially) but it won't hurt to give him some adult pouches now.
Mosi is a slow maturing breed (not as much as maine coons, but somalis aren't fully grown til about 3) and I put him on adult food before he was 1.