The robot will work indoor in an environment with flat ground so this should not be a problem.
I also consider possibility to use some caster like emmeshop.it instead omni ball but I'm little worried about the influcence caster can have on robot.
I think, since it will move on smooth surface, make more sense to take advantages of omni wheel but, since this is my second robot, I'm not so experienced and maybe I wrong to evaluate side effects!