Not sure about that. Mine are maybe 8-10" long and quite chunky but maybe get just one young one as I believe they grow to suit the size of the pond. In the wild they are massive.Blanket weed - Oh yes, we've got lots of that too! Not heard of grass carp, but I think they are going to be too big. The pond is only 6ft square.
One thing that worries me is.......I am about to put some pond plants in but not sure it the carp will eat them as well.