I guess Ill be the bad news bearer, sorry. COngrats on getting your first cichlids ...but...
That tank is already too small for the pleco. In about 3 months, that tank is going to be too small for the oscar by himself. If you dont plan on getting a new tank, Id get rid of the oscar and pleco before you become too attached to them. The bio-load from the waste of the pleco and oscar is going to overload your tank in about...5 days from now probably, unless youre doing 25% water changes every 48 hrs.
Sorry to be the bearer of bad news, but better to know now than later.
Good Luck with whatever you do.
edit: ok, I see you said a 65 in august. In my opinion, thats way too long away if youre going to keep the stock you have and will be too small of a tank if youre keeping everyone. Id re-evaluate what you really want and go from there. If it were me and I had to make the choices, Id either keep the oscar alone or the 2 firemouths and some dithers. For the most part, plecos put out more waste than they consume and poop up the tank. I cant even imagine how true that must be with a 13 inch one. I had a 10 inch in a 125 and it was too much IMO.