I’m fairly certain the number of roids per belt (e.g. colossal, enormous, etc.) is fixed but is the asteroid placement the same as well, i.e. if I warp to a specific rock in one colossal, can I warp to the same one on the next colossal and will the other rocks be in the same place?
No you can’t, while the total number of rocks and total m³ in each type of belts are fixed, their placement on the field is not; nor does the belts spawn in the exact same coordinates in the system. If you want to keep the belt in same position you’d have to leave a single rock behind during downtime; that way the entire belt will be refreshed when you log back in and in the same location.
Not quite true.
For asteroid anomalies, the placement is the same. At least for the system I mine in. The medium asteroid belt always has the same roid placement.
So, as far as I am aware, within anomalies, yes. They are the same.