There are actually quite a few examples of Geonosians fighting in the Arena:
1. The Geonosians that man the sonic cannons.
2. In the defensive circle when Mace says they will not be hostages for Dooku to barter, look at the sky, there are many Geonosians surrounding the Jedi with blasters trained on them.
3. Obi Wan fights off 1 or 2 Geonosians when he's fighting the Reek
4. All the Geonosians that attacked Anakin and Padme in the factory
5. Several Geonosians are seen scampering up to Jedi with either blasters or electropoles, and die for their trouble.
Still and all, Poggle did declare he was going to send his warriors to hide in the mountain, but I guess a few didn't get the message