I wouldn't use flax that was ground previously, all of the 'good stuff' is very volatile and only lasts for a day or two. So, if you use pre-ground flax, you are getting roughage, but very little else. All the desirable properties of the flax would be long gone.
We grind flax on an as needed basis for ourselves and, as of yesterday, started giving a little to our mini, too, thanks to this forum! SE Tex warm weather is coming soon and I'm dying to see what our new little buddy has to offer under his woolies.