With your spey rod and Scandi line I would definitely use a 10' floating polyleader. Attached to the polyleader I'd add anywhere from 6' of mono - for bigger flies, up to size 6 - to 12' of mono for smaller flies.

For now, I'd stay away from casting sinking flies with a Scandi line.

Some casting tips: Scandi lines are prone to blown anchors, so make sure after you execute your lift you still have enough line on the water. If you're standing in waist-deep water you'll end your lift higher than if you're standing knee-deep water.

(I was blowing a lot of anchors until I lowered the height of my lifts.)

I execute my back swing by sweeping the rod tip back in a straight line, then, but before I end my swing, I slightly raise the rod, without changing it's angle. (This upward movement will lift the anchor slightly off the water.) At the end of your back swing take note of where your anchor is pointing, and then make sure you execute your forward cast parallel to and inside of where your anchor is pointing. (If you don't you're fly and line will probably collide.)

Finally, the Scandi forward cast is very short. It's like cracking a whip. I mostly aim slightly upwards.