It can be max 19 lvl difference but the closer the lvls are, the more even distributed the XP is so its always better to get as close in lvls as possible to get good exp on lower lvl chars. a 79 lvl char with a 60 lvl char in party will get like 3x or 4x more XP than the lvl 60 char so ye.
In regards to recharge, if recharge lvl is too low compared to char lvl, it will give 0 mana. This even happens on high lvl because max lvl recharge is lvl 74, if you recharge a guy who is lvl 80, he will get less recharge than normal because he is too high lvl compared to recharge lvl. To increase recharge lvl you will have to enchant it which well, on interlude is rip