My *favorite* classrooms are 3-4 grade mixes (montessori style, 1 room classroom style as well). Montessori is usually 1/2/3 and 3/4/5 & 6/7/8 & 9/10/11/12. One room school house, otoh, is ages 5-18. It's an entirely different method of teaching both academically and socially that ***with teachers trained in that mode*** has absolutely phenomenal results both academically and socially.
So the age thing... not a worry at all. Far from, it would be a huge attraction.
The ASD thing, however, would be concerning to me. It wouldn't absolutely nix it, but it could.
I say this as someone with LFA in my family (my cousin), and who is friends with several Aspies & other HFA people, and one of my son's besties is and Aspie.
If he is prone to violence, it is highly unlikely I would enroll my son in the class. If he was non-violent, merely distracting, I would trial run it. My son is ADHD, so he would go one of two ways... completely nonplussed, or gestapo and not pay attention to anything else. .
I don't think there's a right or wrong answer to your question, I DO believe it depends on your own child and how they would cope or not cope given the individual and the situation.