hello,
so string length is, over all, preference. i like mine to be slightly shorter than to my sholders as i am quite a tall person and it just makes all my tricks a bit hard and slacky. (that might be complete nonsense but basically i like my string slightly shorter, i just feel i have more control.)
sometimes your sticks will hit the ground, it happens. you could shorten your strings and it will happen less, otherwise lift your arm higher?
about the string, you could just buy any old string. it just wouldn't work as well. diabolo string will last longer than any other string (most likely). it not expensive, you can pick up 10m for less than £5 ($8 ish?) an that will last you a good few months.
i often just change my sting when i feel that i am unable to do certain tricks because of them wrapping up. that can a few days if im playing in a wet field (has happened) or it can be a month or two if im only playing indoors and not that much.
when it comes to string length, best sticks, best diabolo's etc it all comes down to preference.
hope that helped
max