I know this reply was old but since the thread was bumped...
My hard water will cause vertical nipples to start dripping within a few days time when in use, the only way to stop them from dripping after a few days is to take them apart and soak the ball bearings and sometimes the entire thing in vinegar or some other deposit remover and reassemble/reinstall... I could fill it up and if left just hanging without use it would not drip, but as soon as it's actually used and the hard water starts to flow through it, forget about it, the scale will build inside the nipple and on the ball bearing and they will start dripping in short...
My horizontal nipples don't suffer this fate, or at least I have not experienced it as of yet...
As for small chicks and their ability to use horizontal nipples, I have had day old chicks figure them out and get them to work, never underestimate the 'pecking' ability of a day old chick, remember they just pecked their selves out of an egg with little to no room to move... With that said I always monitor the day old chicks and verify they are using the nipples if that is going to be their only water source, if I see any having troubles i'll put in an open water bowl with marbles for a few days... I also put in open water for the first day of shipped chicks just so they can more easily and rapidly re-hydrate themselves, then i switch over to the nipples...