Your tank is only 16.9g say 16g with substrate.
that is too small for a Pleco and he will add too much poop.
I would keep testing and changing water to keep tank safe and consider returning/rehoming the Pleco.
You don't "need" an algae eater. I control algae thru photoperiod.
What species of Cory did you get ? They usually need groups and in tank sizes under a 20g Long I use tiny species like C habrosus.
GL with your tank and I hope you have great fun with it.
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