As long as the attenuator is rated to 10 GHz it should be fine. Aeroflex/Weinschel make attenuators of this kind; many others are only rated to 2 GHz or so and these may well allow through-leakage and less attenuation if used well above their frequency rating.
The -30 dBm rating would be the point where unwanted spurious products may appear on-screen, ie the response becomes non-linear.
Input power greater than this would not be harmful until the power reaches +15dBm which would be the level where damage to the mixer diodes is possible.
It's always good practice to always make measurements under -30 dBm and never allow more than 0 dBm into the input of this kind of instrument.