your tournament legal ally choices;
a) codex: daemonhunters
b) codex: witch hunters
c) 0-1 deathwatch killteam. (rules are in WD305 US or WD306 UK)
opponent's permission only allies;
a) kroot mercenaries
b) anything really! as long you and your opponent both agree and use the general rules for including allies as presented in the 'hunter 'dexes, then there's no reason not to!
all the 'offical' rules for including allies are found in codex: witch hunters. (which is slightly different from the earlier codex: daemonhunters version and thus superceeds it!)
cheers!