You should be able to keep a pair of false percula clownfish in a 9-10 gallon tank but the HOB filter might not provide sufficient water movement for an anemone. You would need to consider getting a small wavemaker. Filtration wise, it should be sufficient since you would compliment it with liverock.
For me, my preference is to make my own seawater as I have control over the quality of the water. I have always used tapwater and don't have issues so far.
Personally, I have not tried keeping any clownfish with an anemone before so I do not have any actual experience on this.
This site does list the species of anemones that are compatible with false percula clownfish.