Spinning rod/reel setup is not ideal for float fishing in a river unless the current flow is very slow (ie. slower than walking pace). Because it lacks the free spooling feature, you would need to open and close the bail of your spinning reel several times per drift so line is fed out as the float drifts downstream. Also, the drift would not be smooth as there would be pauses at times when you feed the line out manually. This can be labour intensive, even more frustrating and time consuming than attempting to get used to a baitcasting reel.
That being said, I do use a spinning rod/reel setup for river float fishing from time to time simply because it is something different to try out. It also allows me to change up to a spoon casting setup with ease. There isn't one single best length for this purpose, as everyone has a different body height, arm length and preference on fishing style. Generally look for rods that are between 8.5 to 10.5 feet long. Shorter rods make float fishing more difficult as your float depth is limited by the rod length. Longer rods such as 10.5' allows you to drift better as it keeps more line off the water surface than shorter rods.
If birdnesting is a problem when using a baitcasting reel, perhaps the tension knob when the reel is free spooling needs to be tightened. In addition, starting with a larger float and heavier weight can speed up the learning process. As you become more familiar with your reel, you can downsize your setup and you would be amazed by how smooth the casts can be even with a very tiny amount of weight on.