Don't all Geophagus species dig through the substrate? (correct me if I'm wrong!) So I don't think plants planted in the tank bottom would be very safe especially if they have complex root systems and don't do well with being frequently dug up. That can stress some plants out.
But I imagine floating plants should be alright as well as things like java fern attached to driftwood. AFAIK Geophagus don't destroy/eat plants, just dig them up likely by accident.