There is absolutely no reason at all why you can't slip on final with or without flaps on most light GA aircraft. Bill has a point, you may want to get full flaps down first, but there may be a reason (such as a strong crosswind) that you may want to land with less than full flaps, so go ahead and slip as necessary.
There may be a prohibition against slipping with full flaps in some aircraft, so, obviously, don't do it in those types. Nothing I've flown in the last 40+ years comes to mind. The high-wing Cessna singles often carry a placard "Avoid slips with full flaps extended" or words to that effect. That is NOT a prohibition although I have heard more than one instructor interpret it that way. If you do a full-rudder deflection slip in a C172 or C182 with full flaps, you may experience a rather startling pitch oscillation, but the aircraft will immediately stop doing it if you release the slip inputs. I don't think it is particularly dangerous but it can get your attention which I believe is the reason for the placard. I believe it is due to the high-wing flaps blanking the airflow over the horizontal stabilizer. This is obviously not an issue with a low-wing aircraft.
In regard to the slip being a cross-controlled maneuver, this is obviously correct. However, this is often misinterpreted to mean that if you get too slow in a slip, you will spin. In fact, this is not likely to happen, you can certainly stall (which is a bad thing on final!) but you probably won't spin. The reason is that, in a slip, the aircraft is rolling one way, but yawing the other. If you look at the slip-skid ball and the turn coordinator, you can verify that they are deflected in the same direction (rolling left, ball to left thus yawing right). The aircraft will usually spin in the direction of the yaw, but since it is rolling in the opposite direction the two forces are opposed. I have demonstrated this many times in my Decathlon (at altitude!). Put the airplane into a full-deflection slip and allow it to pitch up to a stall, it simply starts sinking at a rather dramatic rate but does NOT spin.
In a skidding turn, on the other hand, roll and yaw are both in the same direction, so if you stall in a skid you can (and probably will) spin. The classic example is the base-to-final overshoot, with an attempt to rudder the airplane back to the runway causing a stall-spin at low altitude.
Bill's point about pitch control is well-taken. Remember, in a slip, the ASI may not be accurate to the pitot tube being sideways to the relative wind. You need to maintain a constant pitch attitude into and out of the slip. Most GA airplanes will tend to pitch up as you enter the slip, so you will need to add some forward elevator into the slip. If you don't take this out as you roll out, you will pick up an extra 10 kts of airspeed right at the point you are trying to dissipate it.
Probably more than you were looking to hear and I am sure some will disagree with some of the above, those are my opinions! Go ahead and slip all you like,
Cheers, Tony (MCFI-A)