SSCX Only!

Nate you've come to the right(eous) place aka. The Joy. FixedGearCX is indeed possible - and people do it with remarkable results (I for one finished an 8 stage race in one day Sweden based run). When you are that comfortable as you describe riding fixed, you wont be worned down on the 150rpm descends that every course inhabits. But there is a leap you'll have to work on to reach your optimum performance on a cx'ish track - and that is choosing when to get off the bike, whenever the course gets too technical or lack of traction on uphill muddy sections. The goal is not to remain seated on the bike the whole course, but complete in the fastest possible effort.
When you cancel out the technical muddy stuff in your gear-of-choice calculator - you'll probably find your self comfortable between 60-65 GI - dependent on thousand factors.
Keep us posted on how it turns out.

not too worried about dismounting as for the most part it will just be gravel roads with 2 sections of rail trail. ( 2013 Steaming Nostril Start and First Section - YouTube
) I for the most part dont mind pushing a hard gear to gain top end but i do realise that with the possibility of deepish mud that I'm going to want a low gear for pounding through.