Dimage needs pre-flash for metering
> The xi does not support High Speed Sync but the HS does (ie the 5400HS).
In this case, HSS stands for flat pulse flash (the flash fires a rapid series of pulses while a narrow slit travels over the film frame - the shutter is never fully open if the shutter speed is faster than the x-sync speed). It is needed whenever the shutter speed is higher than the X-sync speed.
However, the wireless HSS is simply bad terminology (Minolta's mistake). It means that the shutter speed can be higher than 1/60 s. I've never seen a decent description of this mode - all I know is that it is based on pre-flash metering.
I'm assuming the D7i/Hi is using this mode, because it NEEDS a pre-flash for metering purposes (no dedicated flash sensor).
> Only the D can pre-flash on the 7i/hi.
Exactly. That's why flash metering does not work with older units.
> The wireless feature seems to work similarly on all three HS-D, HS, xi with my 9xi film camera (even with the IR controller).
Because your camera only supports the old (max. 1/60 s) wireless mode. I bet you can see only one flash pulse, the built-in flash and the slaves seem to fire at the same time and only once.
The D7i is different. There's definitely a pre-flash. In fact, more than just one pre-flash - the slave seems to fire twice and the built-in flash two or three times.
> Can one actually do HSS in the wireless mode? The mode selection on the flashes prevent from doing both (ie one has to pick HSS or wireless, but not both!).
I don't think wireless HSS has anything to do with the "real" HSS mode. According to the manual (of the 5600 flash unit) the max. sync speed in this mode is 1/200 s. Interestingly, this is the same as the x-sync speed of the Maxxum/Dynax 7 (the only camera supporting this mode at the time the manual was written). But D7i is able to exceed this limit. It has no trouble syncing at 1/2000 s!
Looks like the wireless HSS mode is based on pre-flash metering, but the main flash is a normal, single pulse. On the other hand, the normal wireless mode is based on pulsed flash - are you confused enough?
It's not possible to use the 5400HS as a wireless slave, because it doesn't support pre-flash metering. However, it will work OK with all Maxxum/Dynax bodies and you can use it along with 5600 units. The manual of the 5600 unit was written a couple of years before the D7(H)i. That's why it (and it's limitations) are not mentioned.