again i would just focus on getting very lean and being very athletic and strong.
crossfit is great for mma you just need to balance it out. if you go nuts w/ crossfit you will be drained for the other aspects of your training. crossfit also takes a lot of calories to recover from and may make it difficult to drop weight. sounds backwards but ive seen it happen a lot.
you just need to put a total plan together that balances all your mma training, strength training, diet and strength.
1-2 days a week of crossfit will definitely help but 2-4x per week might be too much. all depends on what 'crossfit' workouts you are doing, what you are eating, how much you are resting, etc etc
also it is very common for athletes to put on a good amount of muscle in their late 20's