i know its hard to get fish that look good i got a torpedo barb from the pet store that was faded luckily nothing wrong and he is getting colour back but i will suggest look for another pet store a better one if it is this bad, i know pet stores here can be bad in the UK so i just find another one until i like one and i have about 3 i trust and like look around search around.
As techen says yes if you think you lost a fish do your best to find him, as if left it will leave to an ammonia spike i have just lost two tiger barbs no where to be seen absolutely now where
and i don't have many hiding places, but fish will get to hiding places when dead.
To the main questions that people don't seem to answer, pleco's do eat algae but they wont clear your tank from it, so no don't w8 for an algae tank then reply on the pleco, feed it pleco food not cat food, cat food dose not contain algae so you need to feed it pleco food which should contain algae in it don't worry it wont effect you tank
and for Cory's Sterbia cory's are a nice looking fish expensive over here in the UK, also gold/orange or Green laser cory's are the best in my eyes i have 7 of them check my journal out for the pics link in my signature, but again as the people above say please do your research on what type of cory you want, there are hundreds and they all have different needs like different water temp's green and gold laser cory's can cope in a lower temp to sterbia cory's and so on different PH levels and different water hardness levels all can help with whats wrong with that fish and can prevent future deaths, good luck and i hope this of help again check my journal out may help you and if you have any questions don't be afraid to ask
The Cory/pleco you ask about i don't think it is either but it is a catfish, similar to a glass catfish according to my source needs food with vegetable matter so flake or cory pellets should be great
if i am wrong with any of this someone will correct me
i am no expert at the end of the day just hope i can provide good advice
enjoy and hope you don't have any more deaths pic of the tank would be nice as well