Plecos are nocturnal for sure, they like to see you but not to be seen...They also like bubbles too. If you give them a bubble wall in the tank, they will generally migrate to that wall and bask in it...
The reason for the water quality question above is that both fish types in your tank are mega poop machines! In a 55 that equals a lot of water changes and a battle with nitrates. As long as your testing your water (ammonia, nitrites, nitrates) regularly and not overfeeding, thry should be ok for a year or so...But both species WILL outgrow a 55 for sure. I have 3 'common' varieties, an 8" highfin, 9" chocolate albino, and 12" common...the common was 8" a year ago...