a) iMovie HD 6 cannot import from HDV tapes ?
Yes it can, via firewire, but not necessarily in real time.
As you will see from the dropdown menu when you start a new project you get a choice of formats:
DV (which is standard 4:3 ratio) and DV Widescreen, which is 16:9 ratio. Both run at 13GB per hour.
HDV 1080i, which runs at 52GB per hour
HDV 720p, which runs at 34GB per hour
b) iMovie HD 6 can only import form SD tapes ?
No, see above, but not from Hard Disk camcorders, only from tape.