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If you have answers, please help by responding to the unanswered posts.
First off what type of Pleco and what type of algae eater? A common pleco gets way too large for a 20g and chinese algae eaters often just sold as "algae eaters" also get large and aggressive to boot.
You could do 12 neons as they have a low bio-load but if you have the species above you really need to rehome them.