Short track XC and Crits have more similarities than XCO and Crits.
I would not agree that you racing Cat 5/novice (I assume since you are asking the question) would be the best compliment to XCO. A novice crit is going to be 30 minutes. XCO targets between 80-100 minutes. That’s at least double and more than likely triple the amount of time comparing XCO to Crit racing.
XCO is more like a variable wattage time trial with a sprint start, more FTP dependent so you focus on tempo and sweet spot and you compliment that with VO2 work.
A crit is basically an on/off race where when you are “on” you are way over threshold and “off” nicely under, more VO2 max dependent, short explosive power as a primary focus and compliment that with sweet spot and tempo.
Short Track XC and Crits, yes, very similar. XCO and Crits, both have intensity but is a road race comparable to a Fondo because they are both long? They have that similarity but not much else.
Look at the Saturday workouts for each of the TR plans and you will see how they are designed to really hit different energy systems:
XCO MV Week 6 - Saturday - Sweet Spot Focus
The Cross-Country Olympic MTB plans will prepare you for the widely varied and typically high-intensity nature of XC racing by combining a focus on short, highly repeatable bursts of power coupled with efforts bent on driving up the watts and keeping them high for extended lengths of time.
*Notice the sprints at the beginning of the workout
Description - Mount Major +2 begins with 3x5-minute Tempo intervals between 60-85% FTP that each conclude with a 10-second sprint between 180-200% FTP.
The second part of Mount Major +2 consists of 50 minutes of Sweet Spot-ish work between 85-95% FTP with short bouts of aerobic Endurance riding at 55-75% FTP separating each interval.
STXC MV Week 6 - Saturday - Sprint Focus
The demands of Short Track Cross-Country racing blend many of the elements of mountain bike XC racing with those of road criterium racing. Consisting of short, fast dirt circuits reliant as much on skill as fitness, STXC athletes might be best described as the off-road equivalent of criterium racers.
Description - Xalibu +4 consists of 7x5-minute race-like efforts beginning with a 30-second surge at 180% FTP.
This hard start is followed closely by 8x15/15 VO2max repeats where you’ll ride for 15 seconds at 125% FTP and then float for 15 seconds at 88% FTP.
Crit MV Week 6 - Saturday Anaerobic Focus
The Criterium plans will prepare you for the highly dynamic nature of crit racing by placing heavy emphasis on peaking your high, repeatable power output without neglecting your ability to sustain big power for comparably longer durations.
Tour de Nez +2 the most tried and true version of Reno, Nevada’s Tour de Nez criterium course.
This version includes a healthy dose of short, uphill intensity and brief faux-recoveries that force you to push through the fatigue until the actual 30-second rest you’re granted following each 90-second lap.
So sure race crits and it will help you improve your short power that will compliment your ability to attack climbs in XCO races. But I would still structure your training to focus on sweet spot and tempo for the fall season because that is what will matter more in XCO races.
I race both XCO and Crits and have competed and won both race types if that matters.