Rule , if your main has the same skill to lvl 5 it does nothing but if your alt had a lvl 5 skill and your main only had it to lvl 4 it moves up to lvl 5.
So if your alt was a miner and your main had nothing to do with mining after the merge your main can now mine.
Stop selling your alts and just merge them, you paid for the sub and you waited the time why can i carry skills over from toon to toon.