Keel bulbs really require mass.
A roll along the boat's axis (for example, when the wind blows sideways across the boat) is countered in two ways:
The keel bulb moves "upwind", while the "downwind" half of the hull takes on the bulk of the buoyancy. So there's a downforce (the mass of the keel bulb) separated from the upforce (the buoyancy). When the hull is not heeled these forces are in a line and matched, but when rolled they form a "moment arm", as the forces are offset, and together they provide a torque which helps right the boat.
The fin is a handy support for the bulb, but it's mainly there to prevent sideslip - reduce leeway - when sailing upwind.
You mention that the model has a hollow keel. If it's a long keel - large, like a traditional working boat - then I would consider dumping as much lead and scrap iron into it as you can, before topping it off with concrete.
You're right in thinking the heeling forces will be reduced - 1/6th scale is big, and less affected by the scaling effects that drive smaller boats to sail with false keels and/or large bulbs.
Andy