Elections PEI will still accept your ballot if you don't rank all five options.
You can rank 'as few or as many options as you wish'.
So, if you are voting for Proportional Representation as your 1st and 2nd choices, then you could either choose to rank the remaining options as 3, 4 and 5, or you can leave them blank. It's up to you. There is no specific advantage or disadvantage for Proportional Representation if you rank the remaining options or leave them blank.
Why? Because Elections PEI won't look at your 3rd-choice vote unless BOTH your 1st and 2nd options are eliminated.