You will need more then 50 gallons. Common Plecos have been known to get to the size of 2 feet long! I would say something more along the lines of 80 or 90 gallons.
That's a female pearl gourami you have there. I disagree with the poster who said they are aggressive. Pearl gouramis are known for being probably the most peaceful species of gourami - 3spots/blue/gold are much worse. If you decide to keep her, you might like to get a male to go with her - they have much longer finnage, and especially when with a female show gorgeous colours. You can see my male (who I keep with two females) in my av. Great fish IMO - they interact a lot, especially with their feelers. Also, the cories are probably a bit lonely - they like to be in bigger groups, most people recommend 4-6, preferably of the same species but not absolutely necessary.
Obviously you have to sort out your stocking first with the fish you obviously cannot house (I agree with the others, especially about the shark and plec) before you go getting more fish though!
I have one Pearl Gourami (Unfortunately the other passed away), but I can tell you they are more shy and timid rather than aggressive, they are lovely peaceful fish who like gliding slowly along and looking at their reflections.
But seriously they are good, I have a tank slightly smaller than your and my Gourami hasn't grown too big or anything, it's probably smaller than yours.