I have long thought much was overlooked here.
Harry has no bargain granting the Guard. Normally, you bargain with milk, bread, and honey for information. Toot, perhaps out of friendship, perhaps greed, told Harry pizza was even more valued.
Harry had no bargain. He began giving pizza, which is of ultimate value to the Little Folk, without strings. Their acceptance creates an obligation for them to provide something he values even as they value the pizza- so yes, I think the Guard are lifelong feal-bound servants. They value pizza over their lives- they'll go to war for it!- so pledging the lives of some of their own fits. Perhaps it's one per pizza he normally provides.
Wouldn't surprise me to find that each "clan" of the Little Folk supplies a member, and when one falls another grows much larger quickly.
I also think there's an implied joke with their being "dockworkers of the Nevernever" and their power when organized. This is Chicagah, an' 'Arry's Hoffa.
I hadn't really thought if it like that but yes, that does track with what we know of the Fae. At first there was a balance between the pizza provided and what Toot did for Harry. But think of that looooong stretch where Winter was supplying the pizza and Harry was basically MIA and not asking for anything in return. The build up of the imbalance would definitely be a big motivating factor in the little folk responding to Harry. Even Harry felt a bit of an 'itch' at some point when favors weren't balanced between him and someone else. Pure Fae like the little folk would get that in spades. Although I do think Toot does have personal loyalty to Harry. Maybe some of the other little folk do too, due to his rescue of their kin and the way he treats them. But the massive amount he called in BG, some, if not most, were just plain obligated. And being in fear of losing all the pizza didn't hurt any either.