Hum. For once the writers have managed to come up with three aligned options that are not actively stupid.
1. (Good) Take over the Sunrise knights for the main Crusader army, "I don't treat my soldiers as expendable".
2. (Chaotic) Let them decide who they follow.
3. (Lawful) Give them to Regill, but demand his support in return.
The main Crusade can always use more soldiers, to be sure; but... soldiers are expendable. If expending them achieves some goal more valuable than their lives. That's what command is for, to decide what is achievable and what costs are worth paying. And these people have already decided to join the war, to be men under command; presumably they did not do so with the intention of electing warlords by acclamation. There is, actually, a regular chain of command in this war, even if the table of ranks is a bit more informal than what Abramo is used to.
And... the Hellknights' weapons didn't bite on the gargoyles any more than the Sunknights' did; but they did duty as blocking elements, while Lann finished the fight, in an extremely disciplined manner. The Sunknights' archery was much less useful.
Very well, then. The scales come down on alliance and quid-pro-quo, as they so often do: It is worth putting some soldiers of Sarenrae under a much harsher discipline, to gain Regill's aid.