Bettas don't have "friends". They may interact with other fish, but it will either be a territorial instinct (trying to chase them away) or just curiosity.
Your pleco is going to outgrow your ten gallon. At the moment, you are completely stocked with just a betta and a pleco, as plecos have a very large bioload.
The type of fish you can add (once you rehome the pleco) is largely dependent on your water parameters. What is your hardness and pH?