the tank, as a volume isn't fully stocked but i wouldn't add much else with that filtration. as mentioned you wopuld greatly benefit from an external filter, bigger is better. also as mentioned the corries would greatly benefit from being in a bigger group, again bigger is better, but 6 is the said amount for them to be happy. yeah the only pleco that would cause problems is the L081 (small spot golden nugget) which reach around 7" i think, not too big for the tank but a strain on the filtration you have at the moment.
just work towards getting a decent sized external and you'll be on the right track, they aren't hard to fit to any tank and you can pick them up at decent price's, you can even pick up a fluval fx5 on ebay for less than £130 which is really good (if you have £130 lol) but something smaller like a smaller eheim of a fluval 405 or tetratec would be good.
i personally don't think you should add much else at all until you upgrade the filters but if you really felt like you want to then just extend the corydora group, the cryptoheros cutteri cichlids i have are very nice and fairly peacefull and max at about 5-6" so a pair would be suitable, they never bothered my cory's or tiny clown loach's.
