Fermax is similar to Fermaid K, it is a "blended" nutrient with some DAP and some micronutrients, vitamins, etc. There are lots of these blends available, like the Wyeast blend, TheBeveragePeople nutrients, LD Carlson, and so on. Fermaid K is generally preferred on this board because the contents are published and therefore we know exactly what we're adding. Others we can guess, but there is some variability from manufacturer to manufacturer.
"Energizer" is often, though not always, a blend of vitamins and other non-nitrogen compounds. "Nutrient" is pretty much always some sort of nitrogen compound, but those range from DAP to urea to autolyzed yeast. Again, Fermaid is preferred here since we know what it is. DAP is also used frequently to add nitrogen without all the other stuff in the blends.
In general you can use whatever you want, and folks do. A side of beef is apparently a good source of yeast nutrients.
Depending on your recipe and yeast selection, you can have lots of leeway in the amount of nutrients you provide; you might not notice the difference between using Fermaid vs. Fermax. For challenging fermentations or just to get a better handle on the process, it is nice to know exactly what is going into the must; this is where the "we know what's in it" line becomes more important.
