ninjaqutie

I think you should train to the level of the lowest ranking person. Since you are new, your partner should put things back into first gear and go at a pace you are comfortable with and are capable of taking safe (and proper) ukemi. Simply asking them to slow down may fix this problem. If that doesn't work, next time you need a partner, be proactive and snag up someone else.

I think it is also important to note that this person may not have tried to hit you with their elbow. Ukemi often isn't natural and you could have moved in what seemed like an instinctual and safe way, when in actuality you put yourself in a position to get clocked. Ukemi is all about fluidity of movement and body placement.

As far as declining to be uke for practice... not sure if that can be done. You are nage half the time and uke half the time. You can't just do one and not the other. If on the other hand you mean, "Can I decline taking ukemi with this person?" Again, I don't think that is fair. Either ask them to slow down or work with someone else. If the problem persists, talk to your sensei after class.