[[ If the only usb being used on 2012 mac mini, is a usb 3 ssd fast enough as boot drive? ]]
Yes. It will be nearly as fast (or AS FAST) as if you were booting from an internally-connected drive.
[[ Is there ssd software for mac osx to enable trim? ]]
No. At least I haven't found any yet.
Having said that, it doesn't seem to be that much of an issue (or any issue at all). I've been using an Intel 530 series SSD mounted in a plugable.com "lay-flat" USB3/SATA dock for a year now with no issues. Runs fine, boots quickly, sleeps and wakes on command.
If you want to put an SSD into an enclosure or USB3/SATA dock and "boot externally", just pick the right enclosure.