In order for someone to be a training partner, you have to trust that s/he does not really want to hurt you. Accidents happen, but real intent to injure takes things out of the realm of training and into actual self-defense. If you're unsure about someone, simply don't work with that person. Remember, "that person" can vary day to day, depending on what's going on in that person's head and life. In training and in life, we never truly know whom we're dealing with. Be relaxed, but ready.
So, assuming you can trust your training partner to not want to hurt you (while remaining on guard to the possibility that he might), here's the deal:
You can learn from anyone given the right attitude towards training.
If someone is using a lot of brute strength and pressure, use this as an opportunity to defeat your own urge to push back against him. Remember to observe the principles within yourself, stay light and loose, don't resist, and enjoy. You might get hit a lot if you have not yet developed enough sensitivity, balance, looseness, body unity and freedom of action to remain unavailable yet unavoidable in the face of such pressure, but that's okay, your body will learn from it. You're unlikely to get hurt if your training partner means you no ill will. Remember that the person using less strength and resistance gains more from the training, regardless of who's getting hit more or less.
If someone is speeding up, always strive to move just a little slower, maximizing your efficiency. You'll gain from this training, unlike the person who tries to go faster--even if you're getting hit more. Again, attitude. As long as he isn't really injuring you, getting hit doesn't matter in this context. Stay loose and balanced and move with it. It's helping you learn.
If your training partner is much bigger than you, the potential benefits are obvious. Even if it feels overwhelming at first, you're learning how to deal with bigger and stronger people--and odds are, those are the people you'll have to deal with if you're ever attacked! Breathe, relax, and resist the urge to panic. Resist the urge to fight back directly against his strength. That is sure to immobilize you and get you crushed.
If your training partner is much smaller than you, there are also big benefits to be gained. Feel how that person moves. Feel how he deals with--or how he SHOULD deal with--your superior size. Develop empathy to feel what it's like to be this smaller person faced with a bigger one--because one day, for you, the tables might be turned. Pretend the person is much stronger than you are and move accordingly.
Working with a person with far more experience than you have is of course great. Suppress any urge within yourself to impress, please, or "beat" him. Accept all physical and verbal input and enjoy. Of course you'll get hit--that's when you learn! Key is for you to feel as comfortable as possible no matter what's happening--as balanced, relaxed and nonresistant as you can get.
Working with a person with far LESS experience than you is great because it usually offers more unpredictability to adapt to, as well as the opportunity to teach and help someone. One taught, thrice learned. Most of the time when I teach, I'm coming up with explanations and advice on the fly, and I often learn from it. In Guided Chaos, not only does movement come from the subconscious, but teaching can as well. Developing empathy to understand what your training partner is feeling is a huge boost to your sensitivity.
The best way to screw up your own training is to assign emotional attributions to what's going on. Angry that you got hit, either at your training partner or at yourself? You just cut off your learning. Elated or sorrowful that you hit your training partner? You just cut off your learning. Ego-based emotions block learning and make the whole endeavor largely fruitless. The best state of mind to learn in is, in most cases, a state of carefree play, where you're enjoying the entire experience and not caring one way or the other about what happens. Incidentally, Tim related that he had a major breakthrough in his training with John when he one day realized and said out loud, "You know what? I don't care. It doesn't matter." John was pleased.
Another way to screw up your training is to consciously think about and judge what your training partner is doing. If during contact flow you keep thinking, "He's speeding up," "He's using too much force," "Why is he doing that?" etc., you're focused on all the wrong things. Relax and accept your partner's movement for what it is: HUMAN MOVEMENT for you to use for your own benefit. Contact flow is like the solo Guided Chaos exercises, only you have additional input from another person to supercharge your learning. So focus on your learning, not whatever the heck your partner is doing. Tim has a great saying: "It's just movement. It's not personal."
Guided Chaos learning requires a balanced and relaxed mind as well as body. If you disturb and unbalance your mind with distracting thoughts and judgments about yourself and others, you're messing up your own training. YOU are the one who's messing it up, NOT your training partner who may be going a little fast or who may not be experienced enough to verbally "teach" you anything. So long as you maintain a good attitude about your training, you can learn from anyone you can trust not to try to hurt you.
And frankly, once you get some skill and experience under your belt to keep things relatively safe, even those who try to hurt you can offer great learning experiences. . . .
So, train in state of carefree play, remember it's just movement and it doesn't matter, work on yourself and don't worry about others, and enjoy! You'll learn quickly.