I have learned that if the water is warm and you add water about 10 degrees cooler, just a gallon, can cause breeding also. Sounds like the whole tank is covered in eggs, wish my fish will do that, but the only fish that lay eggs for me and live are my Cories and the eggs won't hatch.
You could try it but Rubberlips are harder to keep than most Plecos, i have not had success but many people have, your choice if you want to try. Just remember Rubberlips need fast moving water and smooth stones to lie and graze on.
I believe Gardeni Killifish are from central Africa, but i am not 100% sure. I believe the best ratio is 1 male per 2 females. Not from Africa and can't be kept with other fish, but the Pea Puffer would be a good idea, in a 20 high you could have around 6.