will my mac mini fusion drive work in a 2008 mac pro

Sorry I don't think it will work

Unlike normal HD's or SSD's, the two devices (discreet drives) are married to each other via software & the systems firmware (EFI), sorry I couldn't help my self ;-).

Yes, you can physically move them over to your Mac Pro, but any data on the Fusion Drive will likely get lost in the process of re-installing it in your Mac Pro system (make a full backup if you try). You'll need to use terminal to evoke the Fusion as the BIOS if your Mac Pro supports it (it may not due to its age). I should also point out you will need to be running the same version of OS-X on both in any case. Also your poor fiancé's Mac Mini system will now need a replacement hard drive ;-{

Given all of the work & risks and given the cost of having to buy a new drives HD and/or SSD. I would rather replace the Mac Pro drives Vs the Mac Mini's drives as they are older with a new HD and SSD setup to make a fresh Fusion Drive than swapping things around. You could just do it now by adding a SSD in your Mac Pro now {cheapest solution}. Or, you could get a discreet hybrid drive like a Seagate SSHD or Western Digital Black2 drive which combine the two elements spinning disk & SSD into a single device Vs Apples Fusion drive which is made up of two devices.

Gee, she is going to be your wife man! She'll love you more getting a fusion drive'd Mac Mini! ;-}

If you do give it a try let us know if you succeeded without having to rebuild it.

P.S. Did you see the new Darth Vader Mac Pro coming out this fall, Sweet!