I'm sorry Furby seems to be unwell, it sounds like you've caught her very early so hopefully it'll clear up easily enough.
1. Baytril should do the trick, the dose should be between 0.2ml and 0.4ml twice a day. Bisolvon can help if she sounds quite "chesty", not many vets prescribe this though. Do give a probiotic (Bio-Lapis, Avipro Plus, Pro-C) if possible.
2. Some people are adamant that you must isolate a sick guinea pig. I believe that in some cases it is important, but it is more to do with the condition of the poorly pig than whether the illness is contagious.
I had a sow who suffered two severe respiratory infections; the first time she was isolated until she recovered. However her condition was severe enough that she needed complete rest, her condition had to be monitored exceptionally closely, and she had to be hand-fed aggressively.
The second time she fell ill, I did not isolate her despite all the above applying. She'd been ill for months with dental trouble anyway, and she had lost her closest friend earlier that year, an event she never got over - isolating her would have been more detrimental to her health and I believed the risk to be miniscule that she would pass anything on. As it was she did not pass any illness on to the other guineas.
A small handful of particular respiratory conditions may be contagious, but the majority don't seem to be. Isolating a sick piggie can cause more unnecessary stress; a very ill guinea pig may need separating from a herd just for total rest and time to recuperate, but for milder cases the guinea generally appreciates things being kept as normal as possible.
It is entirely up to you if you choose to isolate Furby. Personally, based only on everything you have described, I wouldn't.
3. Ideally if you can syringe-feed her every 2-4 hours. I found 3 hours was just right. In terms of nights, I give a final feed at 11pm/midnight, and the first feed between 5-7am. You can always serve up some Critical Care on a plate so she can help herself. Do keep tempting her with her favourite foods too, that is important. Also ensure she is getting fluids, I found the easiest trick was to syringe some water between mouthfuls of syringe-food - making sure to give some at the very end of the feed to clean the mouth out a little bit.
Best of luck Eleanor, will keep my fingers crossed for Furby! xx