A breeding pair of the smaller adult geos (ie not Altifrons/Abalios) would work and you could also have some younger ones in there with them.
I had two pairs of orangeheads in a 55g for a while before upgrading, there wasn't ever a problem and one pair bred in that setup.
As stated, the nice thing about these fish with smaller tanks is that they grow slow anyway. So you can get away with a smaller tank for a while if you don't buy the fish as full grown adults.
Just make sure you overdo the filtration and water changes. These guys all like really clean water. The smaller the tank, the more often you have to clean it.