I guess it all depends on how you pronounce Plecostomus. I agree with you its called Pl(eh)co. People who say pl(ee)co must call it a PLEEcostomus. LOL
Here is another example I found on the online dictionary: They are often referred to as Plecostomus, often abbreviated as Plecos or PLECS
The word Plecs said by someone who pronounces it as Pleecos would say PLEEKS I guess. Sounds a little strange to me