Ooh, spa pony set preorder! Awesome.
I actually don't think it would be too random for the set to include both spa ponies and Zecora. Zecora and the first spa pony to appear on the show (the blue one) were introduced in the same episode, Bridle Gossip. Also, if memory serves, the only time one of the spa ponies had a speaking line was in Bridle Gossip, and she was talking to Zecora.