I guess half the otos would be ok too, since you only have a 55gal. That way they won't start "starving", cuz your tank is overclean
. But IMO
you got to get at least those 4 otos, they like to be in groups, are more active and braver than.
Previously: 2x 10gal tank, 45gal aquascaped, 70gal aquascaped
Currently: 16gal FW, 40W CF, ADA Aquasoil Amazonia II/Tahitan Moon Sand, PH 6.5, kH2, gH 4