The simple answer is that it depends on the aircraft. Some aircraft define speeds in calibrated airspeed, and then provide conversion charts. Some define performance in terms of indicated airspeed.
The FAR makes reference only to speed, in knots. If your performance is given in CAS, and this is your reference for Vso, then use CAS. If it's given in KIAS, then use indicated airspeed.
Approach categories are predicated on the potential performance of the airplane, and apply specifically to obstacle clearance protection for circling, and for the missed approach. The clearance criteria is determined based on the potential turning radius of the aircraft at the upper end of each approach category.
It is for this reason that aircraft circling at higher speeds, should use the next higher category.
Note that the clearance is based on speed over the ground; not airspeed. One cannot predict the potential for changes in wind velocity. When obstacle protection is given for circling at category C, for example, radius is computed at a maximum ground speed of 141 knots. An aircraft which is a category C aircraft, but circling at 150 knots, should clearly use category D.
However, a category C aircraft which is circling at 135 knots, with a 15 knot tailwind, is going to exceed the circling maneuvering area for obstacle clearance, and should still use category D minimums for protection. A good rule of thumb is that if indicated airspeed, or ground speed is high enough to warrant the next higher category of approach minimums, then use those minimums. This isn't an issue when landing straight in, but is very important when circling, and when executing a missed approach. Remember that the missed approach is predicated on being initiated at the MAP, established according to the minimums for the category of approach in use. Higher minimums may be required to allow for a wider turn radius on the missed.
Speed may be IAS or CAS, depending on the airplane. However, for cockpit reference, all speeds should be converted to IAS for use.
