It's best to do a group of same species IMHO
. They do recognize each other. I would rehome the Pleco now, before he starts stunting. Plus it's easier to find homes when they are small. He'll need about 125g or bigger.
No rainbow shark.
Otos would fit a 15g if you add some live plants.
You could add a second group of tiny Cories like C pygmaeus. They are super cute and will sit up on leaves.
These were my C habrosus
Make sure you're not throwing away your filter pad every month rinse it in used water to preserve the bacteria. Consider picking up an Aquaclear rated for 30g for better filtration.
Pic of the tank ?
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