I use straight citric acid, but lemon juice also works.
As for yeast nutrients, some do use multivitamins but i believe that's more of a "hey if its good for me its good for my brew" type situation than actual "solid" science behind it, there are better or cheaper options out there.
Kinda two thoughts on it, one is you can over pitch your yeast or add dead yeast for your active yeast to feed on as it will provide all the nutrients the yeast need to survive on. Or the second is to add the nutrients separately whether specific nutrients or other forms, one commonly used "nutrient" is Diammonium phosphate "DAP" as it provides nitrogen and phosphorus for the yeast to feed on. Another commonly used nutrient for home based pure sugar washes is tomato paste as it cheap and easy to find. Personally i typically always do rum washes even if making a neutral spirit as the molasses provides more than enough latents nutrients for the yeast anyways.
But standard "Yeast Energizers" that are easy to find, you need very little of "1 tsp per 5 gals" They're actually typically composed of DAP, old yeast hulls, vitamin B, tricalcium phosphate, magnesium sulphate and the likes. It's typically used for stuck fermentations where the yeast have lagged out.
Now saying all that if your washes are super high sugar concentrations, or are only straight sugar with no extra latent nutrients, you should definitely add some form of nutrients to help along the active yeast. But for most washes that are done with in acceptable ranges <14% and are say corn, rye, molasses, fruit or whatever you typically don't need to add anything as there will be latent nutrients present and at most you may just need to over pitch your yeast amount.
So as long as your not stressing the yeast through high alcohol concentrations or nutrient barren washes you typically don't have anything to worry about.