Re: Training course: switch partner ?
It's really a double edged sword. On one hand, yes, you get all the good experience of dealing with different types of people, which can change how you have to make your defenses (what works on your 4'9" 110lb female partner likely won't have the same effect on the 6'7" 240lb giant of the class).
On the other hand though, as you work with someone, you start to build a trust, which can be a valuable thing in a training environment. You start to know much you up the pressure on your partner, and you feel more comfortable actually attacking them because you know they will make the defense. Then, you switch partners, and now you've got someone massaging your neck because they don't want to actually choke you, or they punch 6" off the side of your face when working inside defenses, and you don't get anything out of it.
I think it is good to develop good training relationships with a few different people of different sizes and statures so you can get the variance in body type without the sacrifice of having to teach someone new how to attack you. I'm also a fan of working the technique with your normal partner, and then doing the "A, B" drill where A's get attacked by B's and then they switch. That way, you get to learn the technique in a controlled environment with someone you are comfortable with, and then are put into the drill and have to adapt it on the fly to new people, like you would in the real world.