Glad the LED bulb worked out for you.

I believe the reason the photocell didn't work is because rather than having an actual relay, it used a solid-state circuit that depends on current flowing through the light bulb at all times. This current isn't enough to light an incandescent bulb during daylight, but it is enough to cause a CFL to flicker.

The screw-in adapters probably aren't even available with a true relay. You'd need to wire in a 3-wire photocell -- these have a true relay and require a neutral connection. A 3-wire photocell will properly power ANYTHING you hook up to it, within its electrical rating. They don't require current flow through the lamp to work. 2-wire photocells, however, are all solid-state devices and depend on current flow through the lamp.

The 2-wire solid-state circuit is cheaper to produce than a 3-wire relay. (There might be 3-wire solid state circuits, too, but they are probably even more to produce.)