You can mix them, but since they are different capacities they will charge and discharge differently, cycling to different depth
of discharge, so placing more of a load on the smaller batteries and probably shortening their lives.
Yes it will work
, and you'll probably never notice. Except you'll be getting less overall life and capacity and probably replacing the smaller ones sooner. If you figure four years is normal, you might never notice that you could have gotten six with a fully matched set.
If the unmatched set is all you can do, that's what you do.