ANStar,
Ryanair and SW operated Classic 737s only whilst their NGs were being delivered (you can't replace a complete fleet overnight) and in FR's case, all the Classics are now gone; they are all-738 with another hundred or two on order.
Likewise Easy still had some 737s from a previous order still to be delivered, when they ordered their A319s (I recall there were no A320s in their original order, but I may be wrong there) - I seem to recall a comment from the then CEO of EZY along the lines that, as directors of a public company, they would've been criminally liable had they not accepted the deal on offer from Airbus, even though the operational preference was for more 737s...
My point is that it is unusual, if you want to mix your routes. If you operate essentially two separate operations, then fine - so you'll never see a
VB jungle jet doing YSSY-YMML in place of a
VB 738; there might also be some strips where you could get an Emb in, but not a 738.
EZY operate all their bases like separate airlines but can move a/c around if necessary - so whilst they were phasing the 737s out, they put all the incoming 319s into the Geneva base (with HB- rego), then did each base in turn, changing the fleet over pretty rapidly in each case.
You're right, some LCCs do operate different types (or variants of the same type) but it adds to the cost base and so starts to erode the 'LC' bit of 'LCC'.
But like I wrote earlier, the less the logic - the greater the probability
.