I had the same 9ft ab rib as river rat. Originally bought it with a 5hp, wouldn't plane off but pushed the boat around for most uses. Replaced it with a 9.9 merc. It was a different boat, quick, it would plane off easily with 2 people and gear. tried pulling a tube with it once, but couldn't get any speed. I now have a roll up 8ft dingy with a 3.5hp it won't plane off, but pushes the boat fine around the marina and from anchorage to shore.
all depends on what your intended use is. If you need something to just to get around the marina in or go from anchorage to the beach in protected waters and don't care about speed. The 6hp is probably fine. If you plan on loading it up with people in gear and going any distance or want speed, you may want to think of more power.