I've read here that I can't mix SAS and SATA2 drives on the same controller - is that really true? I understand I shouldn't mix on the same channel, but the same card!?
Yes. This works fine and is not an uncommon arrangement. You may want to look into using nearline or midline SAS drives instead of SATA for future installations, though. These days, there are fewer reasons to use SATA disks given the high-capacity SAS options available.