What are the breed are the Bucks. My Nigerians go into heat every month if not bred. I think I would go with the smaller ones. I am no expert though. Maybe some seasoned person will chime in.
Personally, I wouldn't breed her until she was at least a year old. The reason for that is because I have found out the hard way that younger doelings have a tendency to freak out when this little, wet, slimy thing starts nosing around for a teat. As far as detecting heat goes, look for frequent urination, tail flagging, mounting other does/being mounted by other does, and being more vocal. Bucks do not have to be in rut to breed.I have a 10 1/2 month old nigerian dwarf doeling who I would like to breed if I can ever figure out when she actually is in heat (a whole other topic).