Your 8' x 15' run is 120 sq feet total. IMHO I wouldn't put more than 2 standard sized ducks in that space. My 3 Golden 300 hybrids have a 10' x 15' (150' sq ft) run and mess it up quite a bit overnight. They also have a huge yard (500+ sq ft) bujt they are confined to the run at night.
My run has 4" sand (with all kinds of little pebble/rocks in it) as flooring on top of a wire base and I am in the process of building a 7' x 12' addition that will be floored with pea gravel instead of sand. I'm adding 5 more ducks (from Metzer Farms) to my flock and want ample confined space for all of them. New run will be 234 sq ft divided by 8 large ducks = 29.25 sq ft per duck.
Now if you considered bantam ducks you could comfortably house a pair and let them have a few ducklings also but I wouldn't house have than 6 adult bantam ducks in that space. Yes, they will survive but happiness and health are extremely important to me and the more space you can give them the better.
BTW, Metzer Farms in Gonzalez CA
http://www.metzerfarms.com/ does offer small duck orders now and you can order as few as TWO ducklings
http://www.metzerfarms.com/MinimumOrders.cfm?CustID=71983. They have a small order charge of $12 (which is really $32 which would include up to 7 ducklings or similiar but read the details at the 2nd link) but for me it was worth paying that and getting only 5 instead of having to order their usual minimum of 10 and having to rehome 5 ducklings. Been there, done that and it's a pain!
It looks like the only bantam ducklings they have though is Mallards. I've dealt with Metzer in the past (7 years ago) when I got my hybrids and they are very friendly and professional people.
I bought a pair of Australian Spotted bantam ducks from Holderread's Waterfowl park last October
http://www.holderreadfarm.com/. They were six months old and are doing fantastic. They are housed in their own yard which is 320 sq ft total which includes a house/run of 60 sq ft. I will let her brood 2-3 ducklings next year (if she wants to that is
)but I probably won't house more than 6-7 adult bantams in this space.
Good luck, keep us posted on what you decide to do. Please PM if you have any further questions on these two waterfowl farms.