1. No, the emb120 is a small aircraft with 30 seats ish. It is much more economical than the 145jet. Those long and thin routes don't bring enough money.
that's reallt 3 different niches, so having both 3 can be useful to cover a broader spectrum of lines.
For long &thin routes, you CAN make money with them, but not as much than bigger shorter ones, and you need to make a strong price micromanagement. If not, you'll be screwed quickly. That's why most players don't bother. I do quite a few of them(Edinburgh - St Johns, for example), but it's a lot of work for a very light benefit.
2.the E170 and E190 can theoretically go that far, but economically they are best 600-1600 nm imo.
Same comment as above, plus that thing : beyong 600NM, the ERJ can suffer from the "too small penalty". So, if demand goes over 80-90 on a 600+ route, it's time to switch to E170.
3. No big penalty, but cost of having another fleet type could be bad depending if you have enough routes for them.
well, third fleet group penalty is really manageable. That's the 4th that can be a killer. What is a bad idea is to have a very small fleet group, especially for the bigger plane of the fleet. Having just 2 or 3 EJets will make them prohibitive. Having 30 on proper lines(600-1600 NM with 70 demand minimum), OTOH, should make more money than it costs. Much more. Even with 2 other fleet groups.