They won't require software or a plugin etc but you might need to change a few parameters in the VFD which control the spindle. However, I think they are close enough in the spec that it would just work without any parameter changes. Even if it did need them it's a simple thing to do.

Regards monitoring the load then this could easily be done using the Analog Output from the VFD connected to a load meter.
But with 8Kw or even 6.5Kw, you won't be getting near the max load with an 8mm tool in MDF @18mm DOC and even if you hit a hard spot the spindle would handle a short amount of overload before it shut down.

If it was me I wouldn't hesitate to fit that spindle or keep it as a spare.