If it was a bottle, it might have been regular Sofie. The Paradisi was a draft only one-off. Sofie is good but the Paradisi was barrel aged with grapefruit. It was outstanding IMO.
I've never used the Belle. I got the pack from NHC so I might as well use it. I love 3724, I'm hoping the belle is at least half as good.
If I were to add the juice of 5 grapefruit, I believe I'd wait until late in secondary when the yeast was done. Depending on how much juice that is volumetricaly, a jolt of ascorbic acid it may affect the ability of the yeast to finish.
That's the plan. I might even just add it to the keg with the zest. I have a few kegs set up with sure-screens.