I must agree with eyeinthesky. It is always best to ask the pilot what ROC/ROD he/she can manage, and then work from there.
I also think "expedite" should not be used to separate two a/c. When the resulting ROC/ROD is not what you had in mind, you will still end up with a loss of separation (and be blamed). A specified ROC/ROD assures your separation and if the pilot can not comply, he/she should tell you, and you can change the plan. If he/she fails to comply without telling you, that's just him/she being unprofessional and dangerous.