In an ideal world, its best to take veterinary advice, but to make sure that the vet in question has experience in treating birds and take it from there. Identifying specific respiratory illnesses is problematic, since many share the same symptoms. Some are caused by allergies, fungus, bacterial infections or viruses.
Yes, quarantining your affected birds is a good idea (and maybe use a different bedding material) and giving the whole flock electrolytes / vitamin supplements will certainly do no harm. If the cause is bacterial or viral, it they will give their immune systems a boost and help fight off whatever the problem is.
Here's a link on respiratory illnesses that may be helpful.
http://edis.ifas.ufl.edu/ps044
I'm sure other members will be a long to offer more advice, but without veterinary diagnosis, treatment is a bit of a stab in the dark.
Good luck