I would agree. If she is 7 months old and has a good body condition, she should be ok to breed. If she isn't "filled out" yet I would wait until she has reached her mature adult weight.
You said you just got her...Maybe it would be a good idea to worm her, give her any of her needed and previously not recieved vacinations,and start her on your feed ration...This would give her a couple of months to adjust and possibly continue to build frame before you introduce her to a billy.
The 8 month 80 pound rule is a good guideline to follow with a boer goat but in this case this particular goat may never reach 80 pounds, Obviously I can't see her frame or potential but any sort of a pygmy cross could very well mature under that weight.
I breed my larger type doelings at 8 mths or 80 lbs. With her being 1/2 I would at least wait to breed her till she is a year old . This way she will mature enough to understand what is going on. If bred to young you might be looking at a train wreck. JMO
If you have not had her long I would wait til she is a year old and this way she has time to settle in, get used to her surroundings and mature a little. Waiting til mid Nov would bring her into spring and it is easier on everyone having a birth after the real cold weather has passed.