There cant be more than one type of creep (At least all data fields I saw didn't suggest that there could be, as everything creep related doesn't allow different creep values)
For the 1x1 cell, create a footprint that sets a 1x1 area to be creep (Just mess around with the options in the applied layer) and then make a dummy unit or something that has that footprint. I *think* that should work. If not, look at how the creep spread behaviors of the creep tumor, hatchery and overlord work.
There is no way to change terrain texture ingame
And you can't use different creeps
You can place flat units on the ground and cover it all, but this will fuck up your minimap and your cpu unless you're playing in a 32x32 map
Also, for all your placeable units (flat textures and creep emitters), set their minimap radius to 0 to prevent your minimap from being screwed up as qwertz said. The ShapeCube model shouldn't cause any lag I believe, unless you are on a really ancient computer. Still, it wouldn't hurt to have them be bigger than 1x1 so that you need less of them