I have a couple of MacMini's, and several other Mac's around the house. My latest addition is the 5K 27" iMac, i7 quad core, 32 Gb RAM, 4 Gb GPU RAM, and a 1 Tb fusion drive. I run almost ever version of OS X. Of all of the OS's, Yosemite is the most troublesome. You need a lot of memory for it to run properly. By a lot of memory, I mean at least 8 Gb, but 16 would be better. Also, Apple still hasn't made it possible to easily go from a newer OS to an older one. I have found only one solution for running, say Snow Leopard, on a Lion or Yosemite mini.
1. Burn an image of Snow Leopard. This image needs to be of an OS currently running on a computer. If copying to a mini, then I would burn an image from a mini. To do this, use the Disk Utility, select the drive with the older OS and click New Image
2. Partition the HD that you will be installing the older OS to so that you have two partitions. Use the Disk Utility, select main HD and select Partition.
3. Move the image to the desktop of the newer OS (it will be faster this way.)
4. Go to the Disk Utility and select one of the partitioned drives and then Restore. Restore: the Source will be your image and Destination will be the "new" drive.
5. Once you have installed a second OS onto your computer then you need to go to the Preferences and select "Startup Disk." It is easy to choose SL from Yosemite, but not the other way around. Also, it is also almost impossible to try and resize a Yosemite made partition from SL.
6. If you cannot select the correct drive for restarting, then restart the computer manually. As soon as you see/hear life of the OS booting-up, then hold down the option key. This will allow you to make an option as to what drive to boot from.
If using a SSD, then I would suggest using it solely as the boot drive and then using an external drive for music storage. I have a 64Gb SSD with two partitions, and a 2 Tb external. The beauty of the external is that I can use it on other computers if I want. I run SL with Amarra on one partition, and have stripped the OS down as much as possible. The other partition runs Lion and Amarra, and is used for "testing." On my iMac I run Amarra with Yosemite with not issues.