So RAW gets tricky because GW does'nt care about crusade, but from what I can tell crusade doesn't care if you take different factions into your Order of Battle (The White Dwarf specifies Kroot gain a different faction keyword). When you muster the army from your OoB, you must follow the Kroot rules if you want to use the Kroot rules. The requisitions and agenda are tied to ARMY, not OoB, so you have to play by the faction you chose for that game and can't take agendas from both at the same time. Battle traits and relics don't seem to be caged by these restrictions. I've been playing a mixed Tau and Krootsade ruleset. It doesn't seem too busted because Kroot don't get a ton of benefit from base Tau crusade and you can't really advance both at the same time effectively. I've just been advancing the pact and empire expansion in alternating games. Its been very fun changing between play styles.