Yes, I’ve raised a few runt kittens over the years, and they need the extra nutrition. Congratulations on getting them this far, it’s no small feat! I would keep giving them formula even if it’s not in a bottle, just mix it up and feed it in a bowl, or I have poured a little over their wet food and they get extra liquid that way plus the nutrition. Goat’s milk is a great alternative and full of probiotics for their gut..plus it’s a lot cheaper. I like the powdered because you can mix up what you want and use it up. The canned which is concentrated and you mix water 1 to 1–you mix the whole can and have to use it up before it goes bad..which takes a while but I like the convenience of the powder. Plus there is the fresh in the refrigerated milk section. You can find the powdered and concentrate in the baking aisle of most grocery stores.
I don’t think your runt is slowing her growth down, but if you are concerned, I would take them for a first checkup to your vet. They will need to be checked anyway and it would give you peace of mind.