FIRST off, KEEP THEM ISOLATED!!  Don't even wear the same shoes or boots when you go between them, or at the very least have a bleach pan to step in to clean and sanitize your boots, but it's easier to just have two pairs IMO…  and wash your hands after being around them and even when going from your old flock to the new hens and after any of them!!!  They're very likely to get all your others sick too, and that may happen anyway.  This is the danger of bringing new chickens in a 'closed' flock, but really, sometimes it's just gotta be.

The runny poo could be so many things, but could also just be stress. 

The scaly mites I recommend getting poultry dust and getting them into a trash bag with it gathered around their necks and shaking the dust all over them through another whole you cut in the middle of the bag so you can fit the shaker canister in there to shake it in… or just put a bunch in the bag and shake the chicken around in it holding the bag gathered around their neck so their head and eyes aren't in there, and supporting them a bit through the bag.  It's rather hilarious to do this, you need at least one other person, the chickens HATE it and won't forgive you soon, but it's good for them!!!  Personally, I'd consider dusting all the hens, even your old ones.  Doing them first, so you are going in the 'right' direction while handling them.

AFTER that, coat the ones with the scaly mites legs with vaseline,  a thick gooy coat of it which will look horrible and collect all sorts of dust and such, but will smother the mites.  You can try just that alone without the dust, but in this case, I'd go both ways.

Some poultry dust says you can eat the eggs, but I skip two weeks, which sounds wasteful, but there ya go.