types within brands

well I know the good brands of food but each brand has quite a few types within it. Like Royal Canin has one specially for Siamese cats, one for indoor nutrition, one for Maine coons, Persians, Sensitive stomachs, Light, hairball, baby cat, growth formula for kittens, indoor beauty and fit, adult formulas, and skin care. I have two traditional siamese cats, so if I were to switch to this food should I do the Siamese formula. That kinda worries me because they are traditional not modern so they are not supposed to be as thin and sleek as the modern ones. One of my cats is only 7 months so does he need to be on a kitten food? I find it confusing. I'm not sure what brand I'm switching to or even if my parents will switch, I just used Royal canin as an example. So what do you think I should feed?

I would advise kitten till a yr old and then whichever formula fits kitty best...

well my other cat is two years old, so should he get Regular adult fod or the Siamese type, or the hairball type?

Regular likely will work .. with RC I suggest readign the labels and compareing fat protein fatty acids and making a decision from that