"My question is can I install OSX on the SSD prior to installing it into the Mac Mini?"
Yes. I would recommend that you do it this way.
"I have a working MBP and a USB enclosure for my drive. Can I just use the installer to install it on the SSD? If so, any special steps that I have to take?"
Just:
- put SSD into enclosure
- "aim" the installer at it
- let the installer "do its thing"
Once the installer is finished, instead of just rebooting while still connected to the MacBook, I suggest you try this instead:
- Power down, DON'T reboot to new OS
- Connect the SSD (in external enclosure) to the Mini
- NOW reboot to the SSD "for the first time"
- Work through the setup assistant, which will ask if you want to migrate you existing accounts, apps, data, settings
- Assuming that this is what you want, give it a try.
When that's done ....
... Reboot from the SSD (external)
Do you get a "good boot"?
Look around -- are things "as you wish them to be"?
If so, it's decision time.
You can go two ways here...
First way (easy):
IF your Mini has USB3 ports, you can leave the SSD in the external enclosure and use it as "an external booter". It will boot fast and run fine. I've been booting and running my late-2012 Mini like this for more than three years, runs great.
Second way (more difficult):
You can open the case and do a "drive swap".
You'll need the right tools.
Be aware that many folks who thought they could do the job, have opened up the Mini, and then.... .... broken something inside.
So... be careful....