Cats typically aren't big water drinkers. Giving them wet food should give them most of fluid they need. It would still be good idea to keep water available if there is not a natural source.
I adopted a stray male and he remained somewhat small and 'scrawny' for the 2 months I had him even tho' he was on an excellent diet. What I noticed about him was that he fairly flew through the air when outside....very active...therefore burning up calories like mad which I think kept him slender. There is also the genetic factor. Your males simply may be a wiry build and will never fill out like some cats do.
Re the weepy eyes. I imagine it is a sign that they have feline herpes. It is very common. Mine had that as well. It was clearing up with l-lysine tabs I got thru' the vet. But the l-lysine was to strengthen kits immune system. I'd just keep cleaning their eyes..and hope they get better with time. My kit was looking lots better when he could get out in the sunshine and run about.
We have a lot of experts on this forum who can help you better than I can. Just wanted to give you a bit of support. The good appetites are super. Kitten need lots of calories.
The $ we spend on animals can get out of hand. Try not to worry. Maybe just cleaning the black ones eyes will relieve their discomfort. Altho' I am not sure they feel bad...it just looks icky to us.
You can also add some additional water to the canned cat food to make a gravy.
You can by l-lysine at any drug store. I get them at the grocery store and grind them up and then add 1/8 tsp and mix it with the canned cat food and water.
If they are out in the country in and around a barn, they will be sure to find water. if there are tanks on the property or other water for the cows, they will surely find it. The little female sounds adorable. Wish I could have her...Tenn isn't that far from Miss, which is where we're temporarily moving to tomorrow for a few months. Maybe I could meet you half-way, lol.