I haven't kept any personally, but at work have often had them instore.
However, I do have interest in them. They are a good selling fish, but you have to show the customers what the adults look like as smaller ones don't have the flare the adults do. They are often fairly coulourful, but its usually in shiny pearly scales like a speckle instead of the bold colours some adults have. They don't seem to grow too fast, but I'm not sure what 3-4 meals a day could do to them. Also they eat food from the sand too. Feed sinking foods. They don't seem messy, infact clean the sand up a bit.
The adults from some species have very long trailing fins. The personality of the babies are "Curious" and Slightly shy, or "cautious" rather. Smaller ones doe great with smaller and slightly larger fish. I have heard the adults are the same, but their size means they still have the potential to do damage if they were so inclined. And I've also been told that when in good sized groups, they keep to themselves mostly.
If you want just one or a couple in a tank, you could go with something as low as 65 gallons. But they do like groups so a 4x2 foot tank or larger would work best.
With Oscars, well, I wouldn't mix them because i feel that the dominant oscar personality would detract from that of the Geo, but it's been done before with good success.
As I said, I havent kept them other than at the store I work at, but this is what I tell interested customers, before telling them to research on their own. Keepers Please correct me if I'm wrong.