I think the original point of pemmican was that it's designed to be a complete food.
Pemmican can sustain a human being indefinitely, without supplementation, if it's made right. The reason beef tallow is called for, is because it contains all the nutrients you need to survive - IF it is proper free range grassfed beef tallow, which has a different (and better) nutritional profile than feedlot tallow. So you'd want to get your hands on the grassfed goodies if you truly wanted to craft your own survival rations for a wilderness adventure or just preparing for a disaster here at home.
I ran across a really good PDF on pemmican (with recipes!) from this board a while back and read it, and I think I may have it saved to my hard drive somewhere also; it was SUPER interesting and maybe someone here will re-post the link to it.
I don't see any reason why you couldn't "practice" with on-hand ingredients, like cheaper cuts of beef and bacon drippings or even coconut oil, before going full-bore with the more expensive grassfed stuff, once you get the process down.