No, one slap pack won't ideal...especially for a RIS. What is your estimated gravity? This calculator can be helpful:
Mr Malty Pitching Rate Calculator
If you pitch with one pack, you'll be underpitching....which can lead to greater lag times, slower ferments, and for your final gravity to not be as low. However, underpitching isn't the end of the world.
On Tap: Barleywine, Coffee Oatmeal Imperial Stout (big big beer)
Fermenting: Double Simcoe IPA
On the Bench: Racer 5 IPA
"One of the first things early caveman did, when he crawled out of the mudd, was to make beer. And cavemen everywhere are still making beer...and drinking beer." - Jean Sheperd