Or is it that they aim to operate an all same single fleet?
I think the only problem in operating the A321 and the A319 is that they have different engine types? Not sure. I know that crewing is not a problem except that there need to be 5 instead of 4 cabin crew as the crew can operate both a/c due to their similarity. I think the cockpits are the same - correct me if I am wrong.
Personally I think the A321 and A319 would be the perfect fleet for the airline but I don't see the point of the A320 which fits in between.
A321 = Bucket and Spade, high capacity routes.
A319 = smaller routes and high frequency routes.