The speed of your fusion drive really depends on the ratio of your SSD to hard drive size and the overall data size on the drives. I have created about 5 fusion drives now all for the purposes of testing and it is not RAID 0 as people assume but Apple's answer to LVM (if you know Linux) and that being said it blows away the competition in terms of performance.
A 128 GB drive with the stock Mini (I assume entry level 500 GB) won't give you much of a performance boost which is why Apple's sales point sucks imo because that's all they are giving you and they are charging a lot. A 240 or 256 GB will give you a substantial performance increase from the 90 MB/s benchmark I got on the 2012 drive alone (far better than 2011 that I just sold) to 250-300 MB/s. If you add 500 GB 840 SSD, now you are talking full SSD speeds as the ratio is 1:1. I tested an install with about 200 GB in data, first on SSD alone and second on fusion and there was no difference. The better thing about fusion besides giving you a 1 TB drive is that all your unused files will automatically be moved to the HDD as your SSD fills up, but until that point everything resides on the SSD.
As for compatibility, any SSD works and you will get much better performance if you put in a larger SSD which you can afford if you don't pay Apple to do it. How to set up a fusion drive
Hope that answers the question.