I've heard that hillstream loaches can be very territorial . . . so I'm not sure how happy several would be sharing a 10 gal. tank! It might depend on the kind of hillstream loach, though. Gastromyzon have a reputation for being fairly territorial. I personally have just one sewellia in a 10 gallon tank with some Endler's livebearers and White Cloud mountain minnows. They seem to all get along pretty well.
So far just the flow from the filter has been enough to keep the sewellia happy, but that may be because the water temperature is fairly low (about 67 degrees). I'm afraid I may have to do something different when the house temperatures go up this summer. (I've only had the hillstream loach for a few months, so he hasn't been through a summer yet.)