Think in most cases asking about the tour manager is just out of interest to see if its someone you've had before.
Certainly i wouldn't have any objections to any of the tour managers i've had on JY holidays although there are 2 or 3 i'd definately like to meet again (well we all have favorites,lol).
Must add there is one TM had with a rival company that i would not want again, all I'll say is he was very unproffesional and its the only time I've not tipped at the end of the holiday.
Numbers arn't a problem to me unless its too small, again with a rival company was in a group of only 7 and had little in common with most of them. Think 12 is the smallest group i'd consider going with, never been that low with JY.
Not a problem to me but believe age range may effect some peoples decision although i like to see a good mix which usually happens.
Mick