How to feed my cats properly?

I have 4 cats I feed them Orijen Dry Food before which is an expensive brand but I noticed it only lasts for 3 weeks the 7.5kg bag of Dry food. I am free feeding them putting a bowl of dry food and they will eat whenever they like. And when the bowl is empty I fill it up always. Am I doing it wrong? How to feed them dry food properly? I can't feed them wet food always because there is no good brand of wet food in my area only friskies, whiskas and fancy feast are available and also one of my cats has rodent ulcer which means she is allergic to certain foods and I found out it's because of tuna or fish flavored canned food that makes her rodent ulcer come back. When I stopped feeding her wet food the rodent ulcer has stopped for about 5 months now and just about 2 days ago I fed her whiskas tuna again and her mouth started to swollen after that is why stopped it again. That is why I can't feed her wet food only. Right now I am feeding them 9lives dry food but I don't have any good results about this brand because one of my cats started to eliminate light green color poop. That is why I want to switch them to Orijen again. And also what do you think about Iams is this also a good dry food for cats? My choice is only Orijen or Iams or Acana because my area also has limited supplies of dry food only Royal canin, friskies, Acana, Orijen, Acana and 9 lives are available. And I read that Royal Canin is not that good either.

I think Iams dry is better than 9Lives dry (which is almost the lowest quality you can get). But it's not great, either. Orijen and Acana are really good foods, but as you know are also very expensive. So if you had to feed Iams sometimes, or mix it with the better food, that would be totally understandable. Royal Canin is about the same quality as Iams. I wouldn't feed Friskies dry or 9Lives dry at all.

Can you get non-fish flavored canned foods? Even Friskies or 9Lives? Cats need to get some moisture from their food, so even a cheap canned food is better than nothing. Some flavors of Friskies and Fancy Feast are't too bad (Friskies Poultry Platter and Fancy Feast Turkey and Giblets are 2 flavors that are considered better than most).

Can I buy iams then mix it with Orijen?

You can. . .I mix several different brands. That way they get the different nutrients and don't get stuck on one brand only.

I bought Iams for now since there is still no stock of Orijen. I have 3 adults and 1 kitten. I bought Iams for adult and Iams for kitten. What should I do now to distribute the cat food to my cats. Should I mix the two or should I put the food to different bowls? What if my kitten will just eat the adult cat food and my adult cat will eat kitten food. Will this be just fine or should I just mix it up together?