Normalization will be useful for the skate clips. Skate sounds are fairly consistent so it will give you cohesive loudness across all of your clips. I'd go for something like -6dB for pop and land tricks. This will give you some headroom for mixing.
Grinds (and super rough ground) can be tricky I guess. They are sustained sounds rather than short transients like pops and landings, and will feel much louder at the same peak level. So unless there is a pop sound determining peak level in the same clip, I'd go for something like -9 to -12dB for grinds.
Most commercially available music is processed for maximum loudness, I wouldn't mess with that. Just pull it down and increase the level until it feels right relative to the skate sounds. I think doing this purely by ear will give you better results than any rule of thumb since the optimal level will differ not only depending on the loudness of the music but also the style and instrumentation.
Just avoid clipping/reds on the main, turn down both skate sounds and music if that happens. If you really want to go down the rabbit hole, you can apply processing like EQ and compression to the skate sounds, but especially the latter can fuck up your sound if not applied with care. In terms of EQ, a low cut filter is easy enough to apply and helps a lot, it removes low-end rumble and will increase headroom that you can mix into.
For context, I have limited experience editing skate videos, but I am an audio professional. I am sure others will have more practical advice, but this is what I'd do from my experience working in sound design and music production.