It doesnt matter. I sometimes put a ST as AMC and play better.
Too if have all my MC's injuried, I put a ST as a MC and no problem.
The bigger problem maybe is in the defense, a ST as a DC/DL/DR increases % of foul and penalty.
Too all depend of hidden skills but I usually put players in "wrong" position and is not a big problem.
I used a AML as ML and i did not notice any different in his performance.