Well, I hate to tell you what you probably already know, but it sounds like you've brought home something nasty with the new birds.
The problem with respiratory diseases is they so often quickly morph right into pneumonia, that is often what kills the birds rather then the disease itself. If your rooster is making gurgling sounds I would strongly suggest starting him and any others who are symptomatic on antibiotic's. And yes, keep them warm, especially at night, until they recover. Probiotic's aren't going to hurt but they aren't going to save your birds in this instance.
Tylan 50 is an excellent antibiotic for respiratory bugs, it can be found at most any decent feed store or
Tractor Supply. It is an injectable but you can give it orally if needles are not your thing. Many of these diseases are viral, some are bacterial. Antibiotic's won't cure a viral disease but it will help prevent nasty secondary bacterial infections.
Keep in mind that whatever this disease is, your birds that survive will be carriers. Many of the symptoms of the different diseases mimic each other so diagnosis depends on testing. Either have a dead bird necropsied or have labs done on a live bird.
Whether it's contagious to humans depends on what it is, most are not but a few are, I believe Newcastle is one of them but that needs double checking!
Good luck with your birds, we've been down that road here, not fun.
... What a mess.
. We want to raise for meat and eggs so we want those to be as organic as possible. Sorry to hear you went through the same, it really sucks. But, live and learn. We will just start fresh. I will try to find a reputable breeder in Canada where I can get fertilized eggs. We purchased an incubator and can't wait to try it anyway 
