Can I set my fps lower than 60 in "60 fps any%"?

I guess as the 60fps gap has been set to prevent some fps dependent glitch.

No, you need to be locked to 60 for the entire run. There are some faster strats that would be done if you were allowed to set it lower.

In particular, the category was made to get away from FPS-dependent tricks. Sadly, the route now includes cube jumps. In particular, a 20 FPS jump in B2 (Higher Ground) would be used for a red L in place of one of the levels in B4.