TRIM had not been supported (got turned on) after Snow Leopard (10.6.8) - I think.
If you still used OS version prior to Snow Leopard - I would suggest to upgrade if you plan to use SSD.
Unfortunately, Apple only support third party SSD (via terminal command) after 10.10.4 --
for my own experience (you can take it with grains of salt) -
if you want TRIM to be supported natively, you might want to get to latest OS your mini can support (I think snow leopard is it).
Search e-bay for Toshiba Apple OEM SSDs (they are SATA-2) - I have been very lucky to acquire them for my 2007 Mac mini and 2009 Mac mini server - without worrying about TRIM issues. Of course, max your memory - is always a good idea since you are going to open the mini anyhow.
Good luck.
Sam