The Lumatek switchables do have a "HQI" setting, though it is not identical to a magnetic HQI ballast outputs. But they will fire Radiums fine on either the 250 or HQI setting. A magnetic HQI ballast will overdrive some bulbs, though quite a few are actually supposed to run on them. Radium and Phoenix are 2 that are supposed to be, but they will run fine on electronics. They just wont be as bright/intense and be bluer. Other bulbs will be overdriven a bit on HQI, and will be whiter and lose life. Radiums run best on HQI, but will work fine-and look better IMO- on the HQI setting on the electronics.
The size of the sockets are the same. Take Lumenmax 2 reflectors....the socket is adjustable to fit 175/250 and 400, the only difference in the bulbs are the length.
If this is the kit you are looking at:
http://www.aquacave.com/lumatek-250w-metal-halide-brretro-kit-with-electronic-ballast-3215.html
The ballast listed as 175/250 will actually have settings for 150/175/250 and HQI( you might want to double check with them...but I am 99.99% sure, I dont know of a ballast that is just 175/250) Consider the Lumenmax Elite or other reflector by sunlight supply...the ords are proprietary and will match. Others will work but you need to splice. Also, you can use the code RC5 at their checkout to knock 5% off. They do have the best prices on that ballast.