Cat food questions like mixing, senior foods, etc.

[BusterKitty]

So my friend got her cat about 5 months ago and I've been trying to help her decide on stuff by researching and going to reception desks and asking about stuff. There's one question that I forgot to ask and it's been stuck in the back of my head for a long time(question will make sense easier if I typed it in situation form):

So Fluffy() is overweight and coughs up a lot of hairballs. Many brands have foods for these problems. Would it be safe to mix the "Weight loss forrmula", "Hairball control formula", and normal "adult" food from the same brand altogether or would that be dangerous for the cat?


Also, when should a cat be switched to senior food(or should it be switched)? My friend says at around 7 years old.

Last one x): Which brand is better: Wellness or Royal Canin?
I'm hoping to get a cat in a few years and I'm going to try and get one from the Toronto Humane Society. I know that they feed their animals Royal Canin foods but my friend's cat is fed with Wellness and it just seems so much better to feed it Wellness since I already know about it. Hopefully, somebody might suggest something even better that's available at most Pet Valu stores across Canada. I will ignore any suggestions of brands that contain wheat gluten or ones that have had ANY products recalled(Iams, Nutro, etc.).

Thank you! ^^