The shield generator on Hoth that the Rebels used was meant to work above ground, but why was it not shielded itself (in case of a ground assault, as in the Battle of Hoth)? Surely the rebels could have had a smaller shield generator to protect the larger main generator?
Yes, they could have small shield generator protecting big one, and even smaller generator protecting small one and so on :)
In fact, shield generator you mentioned protected planetary surface and itself from planetary bombardment reasonably well. Rebels have defense emplacement (trenches and turrets) plus T-47 speeders to protect it from ground assault, and of course ion cannon to allow escape.
All of these defenses worked to a degree. Rebels never had resources to go toe-to-toe with Empire, either in space or on land. They relied on remaining hidden, and if Empire discovers them they would flee like they did in the movie. So, shield generator, ion cannon and ground defenses would just buy them so time to retreat.
As we could see from this other clip, Imperials destroyed shield generator only after they entered the Rebel base. At that point of time it would not matter much if Rebels had one or two smaller shields around because Empire would destroy them too.