If you look at the velocity of the two powders there is really not all that big a difference between the ffg and the fffg. FFFg is a fine grade, burn faster creating more pressures in the barrel. Your rifle might like it. All you can do it try it. I like the FFFg when shooting conicals. FFg shoots sabots just fine for me.

The max load of loose powder in your inline rifle is around 100-120 grains depending on the make of the rifle. I would not go over 120 grains of Triple Se7en. Be sure and check your owners manual for the actual recommendations of the manufacturer.