Do you have live rock or will you be getting live rock? Mechanical filters should not be bought for thier filtration power but for their circulation power. The 330 is fine for a 55 but its only going to effectively cover half the tank since the 55 is so long and narrow. You will need 2 power heads on the other side of the tank blowing towards the 330.
I say it should be bought for its circulation power because established live rock and sand is your filtration. It can handle any resonable amount of bioload placed on it. The 330 will handle oxygenating the water and providing more surface area for bacteria to culture. You can choose to not buy filter pads if you wanted to. I would only do this if you DO have live rock. The filter pads use is to run carbon in your system to absorb impurities in the water occasionally. I've been told carbon should be run once a month at most, ie changing the pads monthly. If you don't use the pads carbon can be bought seperately and placed in there once a month.
Another note is when you purchase a skimmer you should place it on the other end of the tank to increase circulation over there. The two power heads can then be directed to the front of the glass to stir up the front of the tank while the 330 and skimmer pull from the back and refeed into the back of the display. When you do buy power heads try to get 2 that do 200-250 gph shooting for the higher end if possible.
As far as the live fish store telling you this
its very in depth and hard to do in a setting like that. Its good you don't trust the store to some extent but alot of the kids who work there generally know what they are doing. I used to be one of them. If something ever sounds out of wack certainly ask around and always do your homework.