I did exactly as you are planning to do as you can see from my sig.
Sunlight Supply sells female socket adapters for their lumenmax pendants so you can easily hook-up any ballast to them as long as you have a wire stripper tool. I put the connections in an electrical box to be safe, but essentially it's just a matter of matching the same-coloured wires. The lumenmax reflectors are far superior to the standard hamilton ones such that I'm able to get away with using only two pendants over a 6' tank.
The increased PAR you get with mogul HQI over DE HQI depends on the bulb, but it does give you more flexibility as you get good PAR from even higher kelvin bulbs. I went from a 175W Hamilton hood to the 250W HQI so I had more dramatic results.
The 10K XM bulb does give a lot of PAR, but you'll probably need actinic supplementation since it still gives off a hint of yellow on an HQI ballast IME.
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