Clear retainers? Is it okay if I skip ahead ?

Usually the difference in one tray in minimal therefore it wont make a significant difference in the long run, the key is to make sure that you stay on track with you trays as much as possible for optimal results. 

As long as you had the previous tray for at least a week,
you should be okay to go forward to the next one. You don’t want to do this too
often, but once in a while it should not affect your treatment.