I guess that in 200 gallons you could fit about 19 fish, if using the rule of thumb that the first fish needs 20 gallons and then adding 10 for each additional fish (: But I think it could be better to keep a few less there, like 10-15, just so that they would have enough space to move! Since Shubunkins are more like common goldies they enjoy having lots of space for swimming (:
And I think your filter could handle this amount of fish with no problem!
One more thing, I read that when stocking ponds you rather count the surface area and the size of the fish! So a pond without
filtration (not the case for you, so you could add a few more fish) you should keep 2 inches of fish per 1 square foot of surface area. Hope it helped at least a bit (: If all else fails you could try checking it up yourself in a book about pond fish!