I have a late-2009 Mini (not used much anymore). You'd have to give up the optical drive to put a SSD and HDD in the 2009 Mini. You need something like:
http://eshop.macsales.com/item/OWC/DDMMCL0GB/
I don't have the Data Doubler for the Mini but I do have the model for the 2012 MBP and it works fine. You may be able to find cheaper brackets out there but I know that for the 2012 MBP, if you get one with a low-quality connector, you may have issues. If you haven't worked with the 2009 Mini before, it's a pain to open the case but otherwise I don't remember having any difficulties. (A lot of people use a putty knife, but it has to have a stiff blade.)
The late-2009 Mini has the NVidia MCP79 SATA chipset (I'm pretty sure they all do, mine does). There's a problem with some SSD's and the MCP79. The Samsung Evo or Pro 850 (although the Pro would be overdoing it for your use) is fine, as are all the typical Crucial's. SanDisk and SSD's with the Intel Sandforce controller typically have problems. If want to use something else, do a web search for the SSD model and "NVidia MCP79". I would only get a specific model if you can find multiple results where people say that it works with the MCP79 chip (it was used in other Macs as well). You can use a SATA2 SSD (the 2009 Mini doesn't have SATA3), but for the most part they don't produce them any more and they won't necessarily be cheaper.
I prefer not to use a fusion drive (personal preference) and in your case, I'm not sure that it's worth the effort. But I think it should work - you need 10.8.2 (or .3) or higher for the fusion drive. If you go fusion, you really have to make sure you have a good backup plan.
EDIT: Just realized the 2011 iMac would have a 3.5" drive internally - and the iFixit site says in the 21.5" model, they used a non-standard HDD. The Mini uses a 2.5" HDD. If you have a 3.5" drive, you could get a enclosure and use USB, but I would just get a 2.5" HDD for the Mini.