I have an iMac from 2011, Mac OS X 10.6.8, and an SDD in a FW800/USB3 case. Which interface can let me enable TRIM: USB2, USB3, FW800 or Thunderbolt ?
According to OS X Daily, TRIM is enabled for all Apple SSDs, but not for third party drives. The article claims that a third party utility, Trim Enabler, turns on TRIM for third party drives as well. More details can be found on the developer's page. Note that this utility isn't officially supported by Apple, so you should back up before installing the utility.
The interface of the external drive shouldn't matter for TRIM support (USB 2, FW800, or Thunderbolt). However, please note that Macs did not begin to natively support USB 3 until June 2012- with your 2011 iMac you'd be using USB 2 with one of those drives.
Yes, you can enable TRIM on an external SSD. I own a Lacie Rugged USB/Thunderbolt 128 GB drive and I enabled it with TRIM Enabler. I am using it as a boot drive.
I do not know if it is smart to enable TRIM but I could not find any posts where it would stand that TRIM can harm your external SSD.
But for any case, I have multiple backups of my SSD drive.