1. Always have a good play with route planning page before taking your first aircraft, identify the juiciest markets in general terms and what aircraft are suitable. Is the top end of the Short haul market 150-200 or 250-300 pax, the first one I would play E-jets or similar, the other A320/B737 or similar.
2. When I say good play, find the lower end of the market, for example you may see a couple of juicy routes that can take B737/A320 short haul at the top end, but the rest and the most are better suited to something smaller, say E-Jet. Prioritise having the ability to do more that 1 freq per day, you can either do multiple frequencies, or when faced with heavy oversupply on a particular route, you can cut back easily and redeploy an aircraft elsewhere. Try and choose fleets with the view to being flexible and if you can't support a large fleet of the larger aircraft, get the smaller option. You have the game penalty for having more than three fleets, so far better to have 1 good general short haul fleet as opposed to 2 that cover two different short haul niches. Unless you are planning to be solely short haul operations, at which point have the aircraft to suite the niches.