As you point out, 100mm is now the most common. For racing, I still use an 80mm on a hardtail (although for longer races, my FS is starting to be my favorite). During a race, I don't have time to mess around with a lock-out (no bar-mount lock-out, adds weight and I climb in the saddle - so little fork compression), so that feature doesn't matter much to me. If you climb out of the saddle, or on the flats, it can make a difference (if you have long stretches of that where you are).
You can also race with a 120mm (or more), but the geometry of the ride starts getting messed up (same as if you've ever put an older 63mm (like a '99 SID) on a current bike made for a 100mm. This actually works on some current FS designs for 100mm or 120mm where the front end is just WAY too high. I needed something that handled better/faster, so I installed an 80mm and went from a 110 stem to a 135. This had the effect of getting me in that XC-racing position (saddle higher than bars, long and flat over top tube). Keep in mind, bottom bracket height drops when doing this, so pay attention when clearing obstacles. It also lowers your center of gravity, making for a more stable ride and able to corner easier at higher speeds.
I DON'T see this working the other way around for XC racing. That is taking a bike designed for 80mm and placing a 120mm on it. The front will be way too high, placing you in an inefficient position to generate the speed you need for racing. Attacking climbs would be very difficult as it'll be hard to get your weight forward on the steep stuff.
Soooo, to answer your question, you can ride any fork/set-up you wish. Going with an 80mm on something designed for 100mm can improve your racing. Going the other way around, probably won't. Experiment, if you can. Swap forks with friends - as long as there is enough steer tube. Play with stem height by adding and removing spacers. Adjust the saddle - for/aft to see how it affects handling and power generation. Find a loop you can hammer on, time it, see how it feels, and keep doing it - experimenting. You'll eventually get it dialed in to where you'll excel - be it on a 63/80/100/120 or, ahem, RIGID.
Oh yeah, I forgot to mention that the bikes are designed around a specific fork dimension so that the builder can dial-in the steering geometry. Pay attention to that as you experiment with forks because you'll create either a steeper headtube angle (twitchier) or a more lax angle (slower responding). Some bikes that were dogs with the 'intended' angle, become pumas with a different angle.
So, be careful....and make sure everything is torqued right before heading out. Cheers.