Servomyces is simply dead yeast. Prior to being killed, it was fed micronutrients which have been stored in the yeast. There's no harm pitching more into your starter, assuming you then later pitch to 5 gallons or more. If your starter yeast don't use all the nutrients, your main brew certainly will, so there's no harm pitching the entire capsule.
The White Labs FAQ says this about Servomyces
Servo is yeast and is propagated in a micronutrient rich environment
then, and is killed off prior to packaging. Boiling incorporates the
Servo into the wort. The benefit of Servo is that micronutrients, e.g.
zinc, are able to pass through its cell walls to your live yeast cell,
thereby delivering the micronutrients without toxicity. Because this
system is so effective in eliminating autolysis and improving the
health of your yeast, it should be used in every batch.
As to alternatives, I use Wyeast's nutrient blend and put 1/2 tsp, recommended for 5 gallons, in a 2 liter starter. The cell counts have been good. I see no reason why this wouldn't also hold for white lab's nutrient. I think the main concern is flavor thresholds, not toxicity, or osmotic pressure required by the cell to maintain equilibrium.