First of all, although I am not too familar with adster (I never used it) I believe that it is not a contextual advertising program. It is just the "middle man" that helps you sell links (correct me if I am wrong) so there is nothing to worry about that. Further more, the last adsense "TOS update" clearly states that it is now allowed to put adsense and other contextual advertising systems on the same page. I believe that it is only conditioned by the fact that ads from other networks must not be confused with adsense ads. So again, nothing to worry about. Just to be sure, you might want to check with adster policies, but I doubt that there will be a problem.