Ah I didn't know they made 15 and 9 gallon tanks. Well a 9 gallon tank is surprising.
Most people will say that the 15 is overstocked. I also say it is without knowing whether your tank is planted/ what type of filter you have. You should do at least 20% water changes every weekend.
And what is a peltobagrus catfish ? If it's a bagrid it'll prob get large...