A common pleco will get too big for a 55 gallon tank, up to 18"!

, it adds more waste than your tank can handle. A better choice would be a bristlenose pleco, they are great tank cleaners, but don't grow too big. The reason others have asked and are concerned is that all that waste can really raise your nitrates, causing stress on fish, and it will be hard to keep them healthy.
JUST in case the gourami has Hole-in-the-Head, you should use vita-chem . HIH is caused by a vitamin deficiency. It won't hurt to use it on a regular basis of about once a week for all your fish.