I always believe that you should be training the way you fight, but lighter.
How do you crossface someone in their guard? Well anyway...
You should use all techniques, but just do not try to make your partner tap. For example, if I am rolling around and i get a heel hook, i will put the heel in my elbow, and just warn him, "watch for the heel hook". I NEVER crank it, I always let him try to escape while I try to keep him in the same place. Cause I know that if I ever get in that position in a fight, I'm twisting that leg till it comes off, and I'm going to keep it as a trophy(slight exageration). Another example, if I get the armbar from the top, and I am sitting off to his side trying to work his arms apart, ill try hard, but the MOMENT I feel his hands give, and I am about to extend his elbow, I let go. Too many hyper extended elbows because people try to finish their partners in training. So yeah, in my opinion go ahead and cross face, heel hook, ankle lock. Do whatever technique you are going to use, but there are 3 rules I go by.
#1 Try not to use strength, as if to hurt the guy. If your crossfacing, and he moans or something, let up a little, your just training.
#2 Try not to submit your opponent, but get as many really close submissions. For example, I will go from an armbar from guard, I almost make him tap, then I release and try to go for a triangle, almost make him tap, go for a kimura, sweep, anything you want. Transitions are a huge part of grappling.
and number 3, the most important rule in my book
#3 Use common sense, and don't be a douche bag.