community fish are less concerned about other fish species, ... would be like you or i on a farm with lots of horses, ... sure, there would be lots of horses around us, but we'd still be lonely
other fish will fight to the death to be the only one left and enjoy being the only one of their species
some are fine solo and don't care about other fish species, some are solo and don't want any other fish at all around them.
how many otto's do you have ?
have you looked up how many ottos it takes for them to be comfortable & happy ?
don't mean to link to other forum sites, but found this
Keeping Otocinclus catfish in the aquarium | Features | Practical Fishkeeping
"Otocinclus naturally occur in large shoals and are quite sociable, so if you want to try them go for at least six — or more if you have the space and algal growth."
i could guess a smaller tank (10 gallons) 6 of these may be pushing the limits of what you want in the tank
i could guess half of this 3 might push the ottos social limits but could be a very minimum number. if you can do 6 or more, by all means, your fish will love you for it., ... just make sure your tank can handle the bio-load
maybe this is what your mom is concerned about ensuring you have enough of these guys they live happy instead of lonely