What do you mean by hard? Is it the steering which is hard or you feel that the tires with 26psi seems much( causing less traction, bouncing vehicle) while driving.
For 15" rim you should install 185/60 specification tires for best driving, fuel consumption, suspension and steering control. The rim width should be 5.5". The rim offset should be +45. With the specifications I have provided you, I guarantee that all shall be OK. Theses recommendations are for Vitz 2006, 1300 cc.