Powder tends to stain certain colours less and personally I find the powder far more managable to measure.
I noticed you were doing a SS pony, if you're deflocking them do that first or the dye catches in the flocking, and if you're just cleaning them it is best to hold them perpendicular to the water line so gravity does the work and the flock won't stain. Maybe make the pony a makeshift stand so they can dry safest.
I'm sure there are more detailed guides out there, mlp preservation project for instance, but that is the basics.