Yes we switch partners occassionally, sometimes a few times per lesson, but especially when we are approaching a grading, it is generally encouraged to have a regular training partner. It has worked to my advantage in the past when my instructors have given me one-to-one support and training. There are a few guys I train with but sometimes it can be frustrating when you want to establish a partnership for gradings.
I see. I've done it both ways - practice with someone over and over and .... oops, he can't make it to the grading so... um... Who's available? I've also "uke'd" for someone going for his shodan when he didn't have a partner lined up.
Last two of my gradings have had multiple partners without much pre-selection. Vastly different people, too, in the nidan test. The shodan grading was just a queue of 5 guys, one after the other, with the examiner calling out all the requirements.
I'd suggest you make a point of getting used to not being used to anyone - but also being used to everyone. Practice with little folks, big STRONG folks, loose folks, TIGHT folks, fit folks and not so fit folks. Ask them to be relentless and to push, push, push until you can barely stand up. (of course this is very good training for both parties uke and nage because in practice you take turns. work until you NEED rest, and then work until you can't keep going.. harder to do than we think.)