You can make a turn using only ailerons, but it would be a gradual turn with a fair amount of "slip"; it really depends on the plane as to how well it will turn using only ailerons; this goes for models as well as full-scale. A Cub, for instance will sideslip, and the tail will tend to drop to the inside of the turn (adverse yaw) if there is no coordination of rudder with the ailerons. A plane with greater dihedral will tend to turn better using no rudder than one with no or very little dihedral. It's always best to learn how to use the rudder, and to learn to coordinate it with ailerons in a turn. If you're going to fly a tail dragger, it's mandatory to know how to use the rudder in order to take off and land.

As for a Y-harness, sure, it's fine to link your aileron servos to one channel. If you have a computer radio and want to program in differential (put more "up" movement than "down" movement) to decrease adverse yaw, then you have to put the aileron servos on different channels. On my 9C, it's channels 1 and 7. The reasoning behind differential is this: the aileron moving down creates more drag than the one moving up, so it slows that wing's travel through the airstream, causing the plane to yaw to that side slightly. Since this side is the high side, and the plane will normally be turning to the low wing side, the plane is yawing opposite to the direction of the turn. Setting the ailerons so there is less "down" travel creates less drag on that wing, creating less adverse yaw.

On twins, they will tend to turn "better" to the left due to engine torque and P-factor, but the same generally goes for twins as singles as far as using rudder, just more of it in a right hand turn. Some guys will mix the throttle to the rudder, and when a turn is initiated and rudder is input, the "inside" engine will throttle back slightly to assist in the turn. As an interesting aside, in the P-38, which had counter-rotating engines, it did not have the tendency to turn tighter one way than the other, but at high power settings and in combat situations where high speed, tight turns were needed, the pilots developed the tactic of pulling back on the inside engine throttle to help them turn: the -38 could not otherwise turn as tightly as the Focke-Wulfs and Messerschmitts. The downside to using the throttle in tight turns was putting the plane into a usually unrecoverable flat spin if the pilot didn't watch himself.