bio-wheel can increase fish capacity?

i'm a total newbie at this so please don't hurt me ;-)

i received a 21 gallon tank kit as a birthday gift. after getting some basic
advice from two local pet stores, i installed the included undergravel
filter/power head, put in 30lbs of gravel, used water conditioner, add some
bacteria starter/booster and started it running. i waited a couple of days
and then bought 4 goldfish (all orandas: two 2.5-inchers and two
1.5-inchers). the woman at the aquarium store seemed to think that 4
goldfish were fine in there. it's been only two weeks now and i've already
done a partial water change (about 40%) and the goldies seem very, very
happy and healthy so far. now, to my total dismay, i'm reading that it is
recommended to have only 1 goldfish per 10 gallons...ugh, that means i have
two fish too many. i read somewhere that using a bio-wheel will allow me to
maintain more fish in the aquarium since it is so efficient at removing
ammonia. so, my question: if i add a bio-wheel to the system and do a good
water change every two weeks, can i safely keep the 4 goldies i have now? is
there any way i could even *add* 2 or 3 more? i've been reading as much as i
can but i can't seem to find the correct answer for this question. thanks.

my other question is, how do they manage to keep so many fish in the
aquariums at the store?? they all look like 15-gallon models to me and
easily have 8-10 goldies in there.

cb