You can probably safely add them but in a 50 gallon I personally would only add 1 and it would be one of the smaller pleco's that wouldn't outgrow the tank. Bristlenose Pleco's are great and you can get them in long fin and albino (I have a longfin albino bn pleco and it's very active during the day). Also albino bn pleco's are known for being more active during the day so you actually get to see them move. Rubberlip pleco's also will work as well as a couple other smaller ones.