For combos i'd say switch Kabuto out with Tenten GNW
Talents: ?-?-1-2-1 (the first two are decided on what he wants, whether he wants his water main to be full healer 3-2 full Attack 2-3 or a bit of both. 2-2 3-3)
Summon White snake
OR
if he wants AoE Poison switch Kabuto for Karin
Talents ?-?-1-2-2
he will want AF's Poison Tai to stack on Karin's Mystery
Summon: white snake
Since you mentioned Mei in the post
again kabuto need to be switched out for her,
the only stipulationid make is to have Chakra dissection blade on AF and Kyoushi Deer on as summon as this will start a 9 chase combo from anywhere :) hope it helps :D
If he doesn't own nor ao neither mabui then better use mei in the wind empress build with the ninja he owns. As soon as he get mabui or ao he can go water.