That depends which position the hard drive is in. If it's in the upper slot (ie at the 'top' of the enclosure when it's in use) then you will need to remove the logic board.

What size are the screws used to secure the outer casing?

I was surprised to find I could even add a second drive despite going no further than Step 9. (See my account below Step 9.) The second hard drive cradle was much simpler than the one used by iFixit, though. Just the connector, really.

Adding 2nd drive to Late 2012 2.6GHz i7 Mac mini

This is as far as I had to go with disassembly.

Disconnected & eased out original 1TB HD. Removed the two grommets on its side and switched the 1TB HDD with the new 2nd drive cable (not quite the same as the one illustrated here – mine consisted of just the cable and no cradle ). Eased that HD back in and attached it to the previously spare drive socket on the logic board (with drive at the top, right-hand looking down). Screwed 2 grommets into my new 500GB (7mm) SSD, attached old drive cable and eased the SSD in on top of the old drive. Connected that drive to the first logic board socket.

Took a while aligning the antenna plate and the 2 inner screws didn’t seem to line up with the new SSD so in the end I left them out. Everything seems secure, though. Cowling slipped back in place with no problem and that was it.

Currently cloning the old HDD onto my new SSD with Carbon Copy Cloner. So far, so good. Can’t believe how easy it was – just 10 steps rather than 37!