RE: Is ruder required ?
Vikas, the need to use rudder, if the plane is designed for ailerons, is limited to takeoff and landing for the most part.
Ground handling, etc.
A plane with a LOT of dihedral will fly perfectly well rudder-only. Such as a kit-stock SIG Kadet Senior. Or any similar "odl-timer" free flight converted to radio control. It's the dihedral that makes turning with only the rudder possible.
It is common to lower the amount of dihedral when a plane intended for rudder control has ailerons added, if only because the amount of stability provided by the dihedral is now provided by the -pilot-!