I'll be interest in seeing how the corys get along with the pleco - both bottom feeders and I've heard they do well together but also that the pleco get much bigger and could outcompete. Good idea by going with 1. Looks like a lot of fish on paper but adding 1 more shouldn't be a huge difference. Just make sure you get any fry out of that tank if you wanna keep them
Corys and pleco are fine together. I've even had monster size plecos with 1/2 inch corydoras and no issues, even at feeding time.
They are compatible. I would recommend at least 6 corydoras though. You've got enough ground space for more (which I recommend even more). The more the happier they will be and the more you will enjoy them.