Is it safe to use a brass celvis for the throttle? I hear this can cause some interference but I do use 2.4 ghz.

2.4 GHz is not a cure-all. Metal to metal contact will produce so called white noise, which has all frequencies of the spectrum, including 2.4 GHz.
Better play it safe than be sorry afterwards.

Jim Drew says his 2.4GHZ systems are not affected by metal to metal contact noise.
However I agree with PE. The shielding on ignition systems work much better at 2.4GHZ but metal to metal noise generators have no shielding.
My question is why use brass? Use plastic and be sure.