If you put them out, it's definitely going to be safer for them to not be wearing halters. As to putting 2 animals at those ages together, it is really not certain, one way or the other as to whether they will breed or not breed, and if they breed, there is not certain, one way or another, as to whether she will become pregnant as a result.
Not sure if you're intention is to get her pregnant at this point in time or not. Male alpacas have been found to be fertile from as young as 8 months to as late as 4 years old. There is quite a bit of variability in individual animals. As to whether your female is old enough to get pregnant at 18 months, the answer is yes, she most likely can.
You don't say how large your pasture is. If they will be put together, I would urge you to oversee this and plan to be on hand for a few hours to make sure all is well, and that he is not overly chasing and harrassing her or else one or both may become injured. Also, if they do breed and conceive, you will be looking at a November birth. Are you equipped for that?
Bill and Louise Goebel
McArthur, Ohio 45651