Hi,
When flying the metro I believed that the 90kt limitation was related to bog down and air loads on the prop.
Put briefly, in beta/reverse the power lever controls propellor blade angle, so at a higher forward speed full reverse will create more prop torque (drag) for the engine to overcome. Above 90 KIAS I believe that this extra drag can cause engine RPM to decay below the minimum for reverse (98%?), leading the FCU to add more fuel to maintain an on-speed condition. In some circumstances this extra fuel is still not sufficient and the RPM decays further, so the FCU adds more fuel ... end result is an overtemp and usually really nasty engine damage
(This also explains why the 90 kt limit reduces with increased OAT; less engine power, so RPM decay at full reverse will occur at a lower prop torque/drag/KIAS.)
We used to have a restriction to not apply reverse with speeds low for the same reason, and always had to be
very careful when bringing the speed levers from high to low after landing that reverse had been cancelled first.
That said, there's no restriction on applying partial reverse above 90 KIAS, and used to find applying a bit of reverse (maybe 1/4 or so) was useful for slowing the aeroplane quickly from the higher VREFs of a heavy metro 23 (115 KIAS or so).
Regarding FOD, usually find that the props start kicking stuff up on the runway at a fast taxi speed (30-50 KIAS), but never aware of it being a problem at high speed. There's no minimum speed for full reverse that I'm aware of, except that dictated by airmanship.
grade 3