An ssd can yes - that's what I did at the same time as upgrading the ram. As long as it's a 2.5" ssd it should be fine. The Samsung ssd I used was a but thinner than the hard drive that came out but that doesn't affect anything really. You'll need to either have a bootable clone of your drive, or install Sierra from a USB stick you've already prepared (which is what I did).
An excellent guide - many thanks. The logic board was tricksy to get out - the card reader was jamming on the casing, but it came out with care. It's easy to trap the microphone cable and the power button cables when re-assembling, so they're worth looking out for. Successfully replaced the RAM and installed an SSD at the same time - many thanks.
When replacing the PSU (on the re-build) take care not to trap the power button cable behind it - I did...
I didn't disconnect the input cable to the power supply. Because I had the machine lying on its back throughout, I placed it on to the metal bit at the bottom of the machine instead (with a cloth to stop the metal getting scratched). The AC inlet cable was pretty tricky to get un-locked, and the PSU sat on the case fine. It didn't hinder anything else throughout the guide.