1. I have a Yamaha T9.9 Hi Thrust. Smooth, quiet, electric start and tilt, starts every time, uses little fuel, steers the boat in wind and waves, killer reliability- what's not to like? However, the XL is the best length motor on a fixed bracket. You can get away with a long shaft on one of the drop down brackets, but the cowling gets more exposure to the water. The prop needs a protector ring around it to keep the downrigger cable and fishing line out of the prop.
2. In my opinion a fixed bracket, properly mounted is best. Arima makes a great one and I would recommend it. There are other good ones available as well. I believe Garelick who makes adjustable brackets recommends NOT towing with the motor on the bracket. The motor is much more secure being towed on a fixed bracket. My boat came with built in mounting nuts on the port side sponson so it was a no brainer to back out the bolts and slap the bracket in place with some 5200 behind it and as a perimeter seal.
3. On a 21 you can't use the hydraulic steering attachment from the main because the sponsons are too close together and would interfere with the throw of the arm, the 22 Arima is wide enough to use it. Some fellows use TR1, Electro steer and other electronic devices. You can also attach the EZ Steer to the rear of the motors to connect them (which I have) however the difference in sponson/transom alignment make it rather difficult to set up and easily use, so I don't use mine. I just sit on the port sponson, tiller steer and have my downrigger next to me so I'm happy.
4. The T9.9 is about 123 lbs. so it is a load on the aft port side, especially with a full 48 gallon tank of fuel. With a passenger it causes the boat to list to the port. I manage that by adjusting the fuel in my saddle tanks and with the trim tabs. The 150 doesn't care if the weight is on the transom, it gets out of the water just fine but waves will come over the transom into the splash well. Not a problem, the two drain holes let it out and water never has come into the cockpit from the splashwell.