Just make sure you've got something insulating the drive from the metal! I use a small piece of foam in mine.
Careful, foam can bring heat issues! I suggest you put below some of those rubber feet in the HD's corners instead, so ai can flow beneath. When I bought my 1200 the HD was "insulated" with a bit of antistatic plastic, I removed that thing and put the rubber feet on it, and it is much better.
Am I the only one not having that blasted metal shield? I hate it.
Regarding the formatting of the HD in a PC, if you have Win XP or 2000, you can connect the drive to yoru PC and use WinUAE, it suports Amiga drives in its latest versions. keep in ind a 20GB HD is a bit of an overkill (at least IMO, if your amiga is not going to be your 'main' computer), and you will need something like SFS installed (or OS 3.5)/3.9 in it otherwise you're in for trouble.