Hi Davie,
To fish as a full floating set up you would use a floating head and floating polyleader to which you would attach a length or level mono (tippet) in an appropriate diameter for the fly you are using. On a floating set up, the length of tippet could be anything from 6 to about 12'.
You could put a sinking polyleader onto the floating head without a problem, this would effectively turn your floating shooting head into a sink tip set up. With a sinking polyleader you would use about 6' maximum of tippet to prevent the sinking leader from going down and the fly staying up.
For a full sinking set up rather than a sink tip, you wuold change your floating shooting head for a sinking shooting head. This would be fished with a polyleader also and the polyleader should be the same or faster sink rate than your shooting head.
Heads are generally loop to loop connected to your running line so you would not need another spool.
I'm a bit concerned that your 9/10 floating head may bea little light for the rod at 31g. Most 8/9 rated rods are happiest with about 34 grams. 3 grams may not sound like a lot but it will make a huge difference particularly if you are just starting out.
The first heads I bought many years ago were Vision Ace in an 8/9 to go on a #9 rod and they were far too light. I totally wrote off heads for quite while after that. I always buy heads in grams now and totally disregard what rating is on the box
As a general guide these weights of shooting heads tend to suit most rods for most people
10/11 rod 42 - 44g
9/10 rods 38 - 40g
8/9 rods 34 - 36g
If going for "short cut" heads, you may want t drop a gram or two, if going for "long cut" heads you may want to go up a gram or two.
I hope the above makes sense.
Cheers
Ben