I’m a big believer in “Tropes Are Tools” so I think five is a good number for main characters. With thirty-one unique character dynamics, you could fill almost three full seasons (sixty-five episodes) exploring each of those twice. You could even have all five in every season premiere and finale if you only do at least one of those dynamics once. The question is, with three pony races + Spike, what would the fourth be? Bat pony? Qirin? Kelpie? Maybe two dragons; one eastern and one western styled. Breaking down their personalities wouldn’t take much effort with a good five man band template. Just take these building blocks, expand and play with them and we should have a decent show. Slice of life tends to favor a small location pool, usually just one town, where as adventure tends to favor large worlds. Doing both could be hard to justify with a plot device like the cutie map, though.