I don't know where this thing of odd-numbers comes from - a lot of people seem to say it and it simpy isn't true. Fish can't count, so odd or even doesn't matter. I have no idea how this myth got around, but hey ho. :p I have 6 in my sorority and my girls actually love each other. :p
I think you are right, though - if your sorority has been going well with just three, well, you know your fish and how they behave. However, I would maybe invest in a couple of 2-5 gallon storage tubs just in case you need to seperate them as they get older (if one gets sick, the other two may turn on her, for instance).
If I were you, I would sell or rehome the pleco. Those guys have huge bioloads and even the smallest breeds aren't suitable for a ten gal. Bristlenoses (the smallest ones) get 6 inches long and poop like goldfish. Common plecos (the biggest type, frequently sold by uninformed petshop workers are a smaller species) get 2 feet long and poop far worse. Small ones like bristlenoses do better in 20 gallons and for commons you need at least a 75 gallon.