There are actually more cons than pros in my view to operate different brand aircraft for the same route structure. We do that with 737s and A320s and in the future probably with A330 and 787 (currently only A330). Crews are not interchangeable (well, cabin crew is), each family has different seat layouts which means an aircraft change can be a nightmare to accomplish with re-issuing of bordcards and seats etc. As the 737 is quite a bit more fuel efficient it is used more on longer routes, except where the passenger load of the A321 or even of an A333 is needed. Both 737-700 and A319 are pretty similar and used on thin routes.
The complexity of the whole operation is quite staggering though, especially if you have even more subcategories than only the different variants, for example on the boeing fleet we have aircraft with and without winglets (different range and fuel usage), with a fixed ELT or not (some countries require it), with short field performance kit (all new ones, different performance) or not etc.