impereal stout?!?!

i purchased a northern brewer imperial stout kit. i know that its short time but im plan on making this tomorrow morning..... according to the website i can do it in my 6 gal bucket...but my lhbs said that i should split it into to 2 1/2 gal fermentor for the fear of explosion..... my kit includes specialty grains 12 lbs of extract and hops. what do you all think?!? i also have a 11.5g packet of safale s-04, will that be enough yeast? and i also have a packet of safbrew s-33 that i have layin around if i need to double pitch it... im not familiar with yeast ..would this screw up flavor?

You sir, may have the wrong forum...