What would happen if I put on a sportec M1 rear tire and mix it with the stock front tire?

nothing,  you'll be fine

You'll have two different brand tyres.

well, aside from the M1 being likely a 150 causing some wierd handling (particularly when you get some wear in it), the only thing is that the tires have different profiles and will handle differently. depending on the specific tires and their wear this may cause you some concern while riding, or you may not notice. They'll definitely wear at a different rate.

You'll probably notice more differences from the 150 rear than the different brands (assuming you are running a stock width rear wheel). The 150 gets pinched on the rim and gets really rounded at the edges. You'll find it physically impossible to reach the edge of the tire and the bike will feel like it falls (precipitously) into a corner. That gets worse as the center wears. Did that once to me and scared the bejesus out of me. I never rode with a 150 on the gs again.

the M1 is a 150 but I mounting it on a 4.5inch rim. also I will be getting a front soon so I guess I can wait till I get the front. Thanks all