Saab only carries 51 pax when you put HD seats in. Or 48/2, 44/4 if you add C seats. No problems using them right out to 1200 NM, likewise no problems using Q400 out to 1500 NM. You'll make more money with shorter flights, assuming they're full. But both planes are efficient enough that you should make money wherever you fly them.
Fuel use on ATR vs Q400 is actually close enough to identical. You're forgetting that the Q400 is faster, so spends less time to fly the same route. So it burns more fuel per hour, but about the same or even slightly less for the same flight. Same deal for the Saab. Differences are that Saab has 5 min longer turns, and the configs are probably 48/2 for Saab, 60/3 for ATR72, 64/3 for Q400. Plus whatever differences there are in purchase/lease price, and availability.
ATR is more popular because it is around so much earlier, and because it's likely MT/challenge scenarios will start with a lot of them in storage.
All 3 are excellent planes. Q400 is clearly better, but comes much later and is more expensive. ATR offers more flexibility, you can add some cheaper 40 seters if you want to. Saab offers more range than the ATR, and the quicker flights can help cram more in.
As for fleet types, it's not that you need more than 20 in a fleet and then they're worth it. If you stick to 3 or less fleet types, you should have no problems. If you have 4 or more fleet types, it gets more expensive the bigger your airline gets. You need to have a really good reason to run more than 3 fleets. Those reasons are normally because you're doing fleet replacement in a longer game, or you need to get planes faster in order to expand more aggressively, and the extra growth speed, extra revenue outweighs the extra costs.