do I need to add supplements if home-cooked food makes only small percentage of diet?

I'm thinking of trying to slowly introduce my buddy to home-cooked food. It's hard to get the supplements I need locally, so in the meantime, was thinking of just giving him a little bit of it without supplements. Would it be safe to do so if let's say, I gave him a home-cooked meal once a day (he eats 4 meals a day) with no supplements 3 times a week? He has CKD, so I don't want to do anything that'll deplete necessary nutrients.

Also... for carbohydrate sources- what's the best kind of carbohydrate source for cats? I know the best answer is 'none', but if I had to choose, what would be best?

Thanks!

I'm not any help about carbohydrate sources, but yes you can feed up to 15% of the diet as unbalanced.  Since you feed 4 times a day, then 3 meals a week would fit into that just fine.

What are you looking at making?  Just cooking some meats?  Are you looking at balancing it later using a homemade supplement?  I know eggshell has been tooted as a natural phosphorus binder and may be a good option for you.

As you admitted the best carbohydrate is None.
However to answer your question: I'm thinking rice, because for cats wh are having a LOT of digestive problems, a short-term diet is boiled chicken and rice.
If constipation is a problem, you might add a tiny bit of oat bran or cooked sweet potato/pumpkin.
When I first think of carbohydrate of course potatoes or pasta come to mind. But carbohydrates are also found in fruits and vegetables. Just something to keep in mind.
Oh just thought of another carbohydrate: cat grass. A mouse contains some carbohydrates--the grass they eat. Note that while most cats love cat grass (you can grow it or buy it in most pet stores), some love it--and throw it up. Just something else to keep in mind.