Yeast nutrient blends typically contain a mix of trace elements and important molecules including inorganic nitrogen, organic nitrogen, zinc, phosphates and some other stuff that helps yeast grow and complete fermentation. Directions generally call for the addition of 1/2 tsp at the time of pitching.
Energizer is not the same as nutrient - please don't use the lingo or products interchangeably! Energizers are indeed nutrient blends, but typically contain components such as diammonium phosphate, yeast hulls, magnesium sulphate, vitamin B complexes, and tricalcium phosphate. Directions generally call for the addition of 1/4-1/2 tsp at 24hrs, 36hrs, and 48hrs after pitching.
Please be aware that diammonium phosphate (DAP) is toxic to yeast in high concentrations, so be censorious in selecting a nutrient blend for use in starters or at pitching that does not contain DAP. Use DAP-containing nutrients (aka "energizers") in subsequent nutrient feedings (@ 24hrs, 36hrs, and 48hrs).
Free Amino Nitrogen (FAN) is more important to yeast during lag phase than most other nutrients (although lipids are pretty damn important, too). Lucky you, you can actually produce FAN in your mash by resting at 130 degrees F for 20 minutes. An enzyme called Peptidase will break down proteins into free amino nitrogen. However, in fully modified malts, this enzyme has already peptidased stuff during malting, so such a rest is not necessary and can lead to head retention problems. Also, rehydrating dry yeast in water can strip yeast of FAN and lead to poor yeast health during fermentation.
GoFerm and Fermaid K used to be available. Now they are not. In their place are Nutriferm and Nutriferm Advance. Use them the same way you have been using GoFerm and Fermaid K, respectively.
I typically find that addition of yeast nutrient is not often necessary unless I'm brewing a high-adjunct beer or a mead or a wine. However, serially feeding your fermentation nutrients and energizers can help avoid those dirty diaper and rotten veggie smells that malnourished yeast sometimes produce.
Yeast make beer, so please treat them well.