Omega 6 are required like you said. So much of a requirement that the AAFCO has a minimum recommendation. They are Linoleic acid (18:2) and Arachidonic acid (20:4). Both of these acids can be found in regular animal fat. Arachidonic acid is not normally found in plants and cats can't convert linoleic acid to arachidonic acid. Their Arachidonic acid requirement is pretty low. Omega 3's are found in animals. But there are about 9 fatty acids that comprise a good part of the omega 3 category. We, most of the time, are only concerned about 2 of them: EPA (20:5) and DHA (22:6). When people talk about grass fed beef containing good amounts of omega 3's, they are essentially correct. They are wrong when they think that means EPA and DHA. It does contain a very small amount of EPA but no DHA, the rest of the omega 3 acids are: ALA, DPA, and STD. So fat can contain omega 3 fatty acids but not the ones we normally associate with 3's. Chicken fat doesn't really contain omega 3 fatty acids nor does regular beef fat. So diet plays a part
In actual prey the brain, according the the usda cow brain profile, shows it has a good source of DHA and I speculate that the eyes may have a good source of EPA.
IMO it would be best to try and stick with fatty acids that come from animal sources. I can't really give you facts as to why vegetable sources may not be best. Prey can contain on average 3-5% plant material. To me they are prey munching machines. One of the reasons most of us try to stick to a prey model diet.
I don't think that we have one set universal ratio. Conventional vets say 10:1-5:1. I said at least 6.5:1. Someone stated holistic vets are 5:1-3:1 and the same person advised one vet recommended a teaspoon, of salmon oil, per cat per day (which in my opinion is not remotely possible in actual prey even if you averaged them out). Linoleic acid usually helps with dry skin. And I think the 3's help out also. Too much Arachidonic acid can cause problems. But I think you can only run into that problem if you supplementing Arachidonic acid because natural sources seem to be in acceptable ranges. But I don't know if more 3's than 6's could cause problems. Sorry.
Anyway those are my thoughts in a nut shell.
Tools for looking up nutrient info.