Sirrocco
Pedantic
[x] Go with the Ashigaru. Wake up the other three sleeping.
-[x] Inform them than you have Yasuki-sama's permittion to bring two of them to assist you in dealing with a group of ronin, in order to retrieve some stolen object. There's no honor in lying, but if posible, try to not give a lot of details.
--[x] If is absolutely neccesary, inform them that you have Yasuki-sama's trust in your word as well.
[x] Let them a moment to prepare themselves, and to decide who will come with you. In the meanwhile, aproach the kitsune.
-[x] Inform her that you got the approvation of your lord, and he even provided you with some assist.
-[x] "Once we're ready, please lead us."
So, it's decent to begin with. I'm curious as to why you're trying not to give a lot of details. I suspect you have some reasoning, but I'm missing it. I figure we wouldn't want to spring it on them while we're haranguing the ronin - otherwise, their responses might give us away. Let's suppose we tell them know something like the following:
"A spirit ally of the Crab has had something precious to them stolen, and has requested our aid. I intend to go retrieve it from the pack of ronin currently guarding it. Yasuki-sama has given me leave to take two of you to assist me in this matter. Hopefully I can bluff them a bit, and get it without bloodshed, but if the swords come out, I will need you to watch my back."
That tells them that the better they can back our bluff, the less likely they are to see combat, but that whoever comes with us needs to be able to fight, just in case (and would be better as a defensive fighter, rather than an offensive fighter). It also tells them that spirits are involved, and that we have Goro's blessing. This helps them set their motivation, and also helps them pick out which among them can best assist us.
...and yes. In retrospect, we should have taken time to get to know the ashigaru. Let us both fix this issue as soon as practical, and take this as a lesson for the future.
I also think we should include something about asking the ashigaru who they are and what they're good at while en route. We have no need for stealth, and knowing a bit more about them will likely help us refine our strategy before we arrive.