Work for Hire: Once you pay them, they act as part of your fortress, until the contract expires. After that, they either renegotiate, or wander off. They can be military or civilians.
Actually, I think this might work much better if we use this as part of a way to reduce the immigrant floods.
Part of the things Toady has said is that he wants it to be possible for dwarves to decide to just emmigrate out of your fort, and out to the villages around it, so that you have to have a nicer place for dwarves to actually want to live there.
(Of course, if we are speaking about having more complex ideas of what a "nicer place" is, there's always
Class Warfare and Pursuit of Happiness.)
So, instead of having massive floods of immigrants, we get few people who just want to move in, unless maybe they're family of someone already living and working in your fort who has made a success of themselves, and encouraged their family to come join them.
Other than that, you have itenerant workers who may come, especially if you have plenty of industry going on, (as in, they only come if they hear you have plenty of jobs, and they are out of work) looking for jobs on contract. You have to pay them to keep them, but they may just settle down if they like it there.
If you DON'T want them, you can just refuse to contract them, and they walk away. No need to be forced to deal with overly abundant immigrants.
Something similar can happen with refugees, where they only come if you have proven your fortress to be a very safe place, especially by being capable of slaughtering any ambush or seige that comes your way. Refugees may be temporary, or you might be able to put some to work and get them to stay.
Actual members of your fortress ("Citizens"?) would be the ones who could work on Company Credit. Foreign dwarves would want coin, or at least bread, for their work. (This assumes that Fortress economics follow a somewhat
Corporatist model, like a mining company, where employees of the company get their food and supplies from company stores, and live on company credit accounts.)