Turians and quarians are both dextro-amino based species, so they're probably more compatible with each other than anything else. I don't think a turian-quarian hybrid is possible but if the quarian's shitty immune system is compensated for I don't see any obvious reason why they wouldn't be compatible. I'd even go so far as to suggest the possibility of a common evolutionary origin - they've got the exact same "doglegs", similar hands, etc.