George,

trying to mix in a room that is basically 12'x10'x8' ... These have made
the room a place where I can actually record without hearing the room in the
mics. But for mixing, I'm not sure.

If you're not miking low frequency instruments then you're probaby okay
recordign in a room treated only with thin foam partitions. But you're
correct to be concerned about mixing accurately, especially in a room that
size.

Have a look at the Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

You'll also find several other articles about acoustics on that page.

Is it possible to make a room like this a place that you can get an
accurate mix, particularly in the 50-250Hz range?

Accurate is relative. But with enough bass trapping you can definitely make
a room that size useable, and create reasonably portable mixes.

--Ethan