I have always used Aptamil for my LO who is now 13 months. The first stage milk should be ok and shouldnt make her constipated as babies are normally looser on Aptamil than other brands and is often recommended when other brands arent agreeing with baby. I wouldnt mix them though and think it would be best to just change when the box is empty. Be aware that it may still upset her tummy for a day or so while she gets used to it, although seeing as they are the same brand, she should be ok.
Having said that, I wouldnt change without speaking to your HV first, just to make sure. My LO never had as much as was on the guideline either but it is just a guideline so go with what suits your baby.