Variety and amount to feed?

Based on the recommendation of several friends, I'm feeding Porter (Ten year old shelter kitty) a half cup of Orijen Six Fish dry food per day, and he receives a quarter can of Wellness Core Salmon flavor twice a day. He's recently had dental surgery, the shelter extracted one of his canines, so I am feeding a pate type food because I thought that would be easier for him to manage while he figures out how to eat around that missing tooth, but it seems like a flaked meat sort of canned food would be better in the long term. Is that correct? 

I'm concerned that I'm either over feeding, or that he's bored with the food he's getting. I'm not really familiar with the appropriate feeding for canned food at all, so I would really appreciate some help with this one. Am I feeding enough or too much?

Would giving him a variety be better for him? He seems to really like the fish flavored food, but the last couple of days, he's seemed to be more interested in the dry stuff and is leaving about half of the wet behind. I have thought about picking up the chicken and turkey flavored cans of Wellness Core to change things up a bit, but I know that if I did that with my dogs, they would get diarrhea from the change. Is variety okay for cats? 

Thanks for your help in educating this newbie! Porter thanks you too!

Here are some good articles for you to read on nutrition for cats.

http://www.thecatsite.com/a/choosing-the-right-food-for-your-cat

http://www.thecatsite.com/a/how-much-food-should-i-feed-my-cat

Also, you are feeding a lot of fish, which is not recommended as the main diet. Here are some reasons why.

http://www.littlebigcat.com/nutrition/why-fish-is-dangerous-for-cats/

That is true, I am feeding him fish. I have a dog that is allergic to chicken and another one that is absolutely intolerant of turkey and corn. They are on a salmon and tapioca based dog food. I chose Porter's food, by avoiding ingredients that my dogs are allergic to, because sooner or later, they WILL get into the cat food, (they're Labradors, food is their life) and I don't want to have to sleep by the back door when that happens. 

I have very limited options on what I can feed as a primary protein source because I don't want sick pets so I'm in a lesser of two evils kind of situation. I think I can get away with rotating his canned proteins though, because he seems to eat the canned food really quickly and snacks on the dry off and on throughout the day, which is fine by me, but I wanted to know if I could go out and get him some canned turkey or chicken based food, or whatever happened to look interesting to me at the time, without causing Porter a bunch of digestive upset. If you switch up proteins frequently on dogs, they get diarrhea. I did not want to make Porter sick by experimenting with his diet. This poor kitty has been through enough without my adding that to his troubles too, you know?

I appreciate the links! It's always good to have more information! 

Generally, you can rotate different flavors of wet food without issues. It's switching up dry foods that causes problems. However, each cat is an individual, so I would definitely watch for any signs of digestive upset.

Oh, and these days, you have many more non-fish protein options than just chicken or turkey. There's beef, pork, lamb, pheasant, duck, venison, even some more exotic ones like brushtail and kangaroo.

In my experience with rescuing poorly bred Labradors (I've been rescuing dogs for 16 years), an allergy that starts out with chicken, eventually translates to all poultry, so I just avoid all bird from the outset. I have looked into foods with more novel proteins for my dogs, but I cannot get them consistently where I live and the kibble manufacturers that do consistently produce a novel protein product, often source that protein in China. Chinese meats are cheap, even their human grade stuff is less expensive than a US source. Every pet food recall I have seen that was not related to poor sanitation practices at the US facility, had its roots in ingredients tainted at the source in China, usually due to feeding the animals antibiotics that are banned in the US for a good reason, so I avoid foods that are made in China and try to purchase sourced in the USA products only. I feed my dogs Nature's Variety Instinct Salmon Meal, which sources its ingredients for that kibble in the USA. Porter's Orijen is sourced entirely in Canada. 

I'm sure that how a cat's allergies present and how their progression happens is entirely different for cats, and I wish there was the wealth of information easily available about cat food, that I can gather about dog food. I wish dog food advisor would finish it's cat food advisor website... that would be so helpful!