Alternatives to bottles...
1) never have used nipple buckets, but they look like a pain to clean!
2) don't bother with any sort of bottle after 3 days old. i bottle feed for the first 3-4 days to make sure they get colostrum and a good start. once they are wolfing that bottle down, usually 3-5 days, they learn to drink from a pail. i've found they figure out how to drink in a feeding or two. i've also found if you don't stand over them and baby them while bucket training them they figure it out better. if there on the bottle for weeks they seem to take longer to adapt to the pail. however, any type of feeding system is going to be a pain in the hind quarters with the calves in a single pen. sucking each other is not good either especially if you plan to use these crittters as milkers or breeders. it also spreads diseases.
The yellow, runny stool is normal in young calves.
Oh and be sure to have calf starter available to them!