SLS/PE3 Question

Side To Side Squat is somewhat a combination of PE2 and horse stance.

Is it fine to add (2 to 3 sets)X1min horse stance to my WU so that I can get my glutes closer to the ground in PE3?

I'm sure you could add this in, yes. But, I don't think it necessary. I've found that even though I want to do extra work to get there faster, my body often resists. The body will take as long as it takes. I've found that by going harder on the work sets, as Zach says to "suck it the f--- up" that I get more out of them. As a result, extra work sets or auxiliary movements don't have as much priority.