Lots of the fillers people use might give the aesthetic look they are going for but have adverse effects on strength and performance. This can be exacerbated by the type of epoxy used and the mixing process. A proper filler will make the fillet tougher and reduce tensile stresses due to resin shrinkage on cure (the crack you see on unfilled epoxy thick fillets). For fillets you want good toughness and peel strength. Micro balloons (especially hand mixed) will reduce the peel strength (look at T-peel), tensile strength and toughness.
Laminating resins are designed to be thin, wet out dry fabric, and have good inter laminar strength and reasonable modulus. They dont have good toughness, or peel strength.
Structural Adhesives use different hardeners, additives, and fillers than laminating resins. The ingredients depend on their application and can include spherical fillers, short fiber (ground) ceramic, longer fiber or micro-balloons.
Micro balloons are there to lighten weight and to thicken, not to increase strength or toughness. For example we used to vacuum mix micro-balloons with epoxy adhesives for closeouts in the edges of honeycomb panels, or to fix damage in honeycomb panels. One other thing we use micro-balloons for is bondline control. A small amount of micro-balloons mixed into the adhesive maintains a certain minimum bond line (keeps the adhesive from running out).
Silica (or ceramic) filled adhesives are stiff, have good shear strength, low shrinkage during cure, and have poor peel and toughness, They can be thixotropic if that is important. At the day job we use ceramic filled adhesives in areas where we have pure shear, like bonding a cup over a shaft.
High temperature resins have a high degree of cross-linking, good for temperature bad for toughness.
For almost all rocket applications (including fillets) you want a tough adhesive (high elongation), with good peel strength a the temperature you need. A good example is 3m DP-420 Black. it has a T-peel of like 60 lb/in at RT and like 20 lb/in at 180F. This combined with an overlap shear strength of over 3,000 psi at 180F.
My suggestion, for highly loaded rockets, dont use micro balloons in fillets, might look good but not giving you the strength and toughness you need. Use a real toughened resin.
Mike (worked for 30 years in composites) K