I am much further south of you.

Our rule of thumb is that you need to plant alfalfa at least 6 weeks before fall killing frost. 

You probably have more leeway if you are planting a pure grass stand.  But planting alfalfa now - with a killing frost the end of September - would concern me.

Will fall cereal rye grow in that kind of soil?  or maybe wait until spring and plant a spring barley,wheat or oats crop. Then plant an alfalfa-grass stand after harvesting spring barley, wheat or oats?

That is my comment.

P.S. I presume you have animals to feed your crops to?