FMF had three types of race frames not just chromoly. They had chromoly, aluminum and an aluminum lug/carbon fiber tube.
Their forks were either chromoly or carbon fiber. Their chromoly forks were heavy for a race fork and would put up with a fair bit of abuse. What differentiates a race fork from a lot of freestyle forks is there are no brake mounts and the only option is a 3/8 axle. There are some very light race forks but this is not one of them.

Chromoly race frames are no more hollow than any other chromoly frame, they are all made out of tubes, a bike made out of anything but would be too heavy to ride. That said, a race frame may be double or triple butted chromoly where a freestyle frame may not be butted at all (straight guage). Butting refers to a tube where the wall thickness varies, the tube will usually be thicker at the ends where it is being welded and then thin in the middle for weight savings. These tubes are as strong as straight guage when they are undamaged but I would think that they would dent a lot easier.

There is no reason you cannot use a race frame for other styles of riding. BMX racing is not all done on flowing tracks with tables and doubles. At the higher levels much of the race is in the air and these frames/forks stand up to a lot of failed attempts at clearing pro sections, which are essentially huge dirt gaps. The comment about grinding is accurate, the chainstays and dropouts are not as beefy as on other style frames and the rear dropout would normally not accommodate pegs.
You may find the frame geometry odd for doing tricks on, a lot of race frames do not allow you to move the wheel up as far as you may want to, this will make the front end harder to lift. In addition, they will often not accommodate a very large rear tire without rubbing.