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To reach the optimum max rpm of lower turn motors (high kv), you need lower voltage or they'll spin too fast and overheat. However, to get high power at the lower voltage, you need high current. Higher currents are inefficient and are harder on all electronics and batteries. So, most people prefer a higher voltage system coupled with a lower kv motor. Power is the same, but efficiency is higher, and higher-end batteries aren't as important.
5s2p A123s would be a good choice for an early MM. But, I would probably go with a 9XL instead since it is a closer match kv-wise for the voltage you want to use. For gearing help, you can calculate an estimate of top speed using
these web pages.
I wouldn't worry about stressing the motor with first to second gear snap. The drivetrain is already in motion when it shifts, so it's not the same force as when it's trying to accelerate from 0mph. If anything, the stress may be too much for the arm and lever design of the clutch system. And, the Tekno clutch system only works from 0rpm up to the engaging rpm - it's still fully engaged during 1st to 2nd gear shifting so it has no effect.