Although I don't have goats, I have been interested in this conversation.
The following may or may not be relevant...
Up here the only real difference between 16% cattle and 16% sheep feed is the copper. I haven't asked the question but I imagine the same is true for 18% feeds. (I get a 20% parlour mix for Hillie; I don't think there is a 20% sheep feed - maybe Ram & Lamb Mix?)
Copper, although a necessary trace element for all species, is thought to be poisonous to sheep in large quantities. Some breeds, specifically Texels, are particularly sensitive to high levels of copper and there have been some cases of death from burst livers in Texels given copper.
In these parts, all the ground is highly deficient in copper and the farmers hereabouts have found that copper in feed and drenches doesn't harm Texels on this ground. We would all be careful not to be drenching Texels with a copper-containing drench and at the same time giving a feed containing added copper, though.
It seems that other breeds of sheep can take a lot more copper than Texels.
So firstly, do of course check the specific cattle feed you are using for your goats, but it may be that it would only really be harmful for Texel-type sheep and possibly not even then if your ground is highly copper-deficient.
I am also wondering whether an alternative to using cattle feed for your goats would be to use the equivalent (same protein %) sheep feed and, if the goats seem to need extra copper, to give them a chelated copper drench, or some other copper supplement, as appropriate?