Hillstream's like strong current, smoothish rocks to hang onto, and relatively cool temps. They also like their own company, like many loaches. The kuhlis will like current also, don't mind cool temps, love plenty of rocks and wood to explore and hide under.
Both will suit a soft sandy substrate too. Try looking at 'loaches online'.. best site for loach info out there, have a great article on hillstream loaches too.
Edit. They aren't going to fight or anything, probably pretty much ignore each other.. I'd be more concerned the hillstream might not fare so well as a single fish. I haven't kept any, so I am not sure. Kuhlis I keep, and if you want to watch some play, get some Black ones. They are more outgoing than the striped ones tend to be, often do what I call the ferris wheel. Three or four will chase each other in vertical circles, over and over and over.. seem to be having fun. Stay out in the day. Both types like a lot of plants, I've seen my striped kuhlis hanging in a thick tangle of floating plants, seeming as though they were hanging out at a spa. In the day time too.