I've mostly used Trail Boss with 70-80% case capacity loads and never had ignition problems. I've tried some .45-70 loads down to maybe 50-60% capacity. Several people have reported with lighter loads (closer to 30-50% case capacity?) there can be ignition problems with powder position in the case (especially shooting downhill). I've never tried this myself. A filler would keep the powder against the primer and should prevent ignition problems. If you're looking for exceptionally light loads, mostly subsonic, you may have to use a filler.
I use Unique for most of my reduced loads (opened my last bottle a little while ago, I'm rationing since it's all I have) but have also tried 800X, Bluedot, Reddot, Titegroup, IMR4227, Longshot, HS-6, and probably some I'm forgetting. With conventional pistol and shotgun powders (not Trail Boss) I've never had issues with loads as light as 8-10gr which I guess is around ~20% case volume charge in many common medium rifle cases (.308, .303B, 7.62x54R, 6.5x55, etc.). I've used both magnum and standard rifle primers and not noticed a difference.
I use Unique for most of my reduced loads (opened my last bottle a little while ago, I'm rationing since it's all I have) but have also tried 800X, Bluedot, Reddot, Titegroup, IMR4227, Longshot, HS-6, and probably some I'm forgetting. With conventional pistol and shotgun powders (not Trail Boss) I've never had issues with loads as light as 8-10gr which I guess is around ~20% case volume charge in many common medium rifle cases (.308, .303B, 7.62x54R, 6.5x55, etc.). I've used both magnum and standard rifle primers and not noticed a difference.