Back in the early days it wasn't unusual to see even some top name drummers using a mixture of brands. This shows up in a lot of vintage photographs. My guess is that back then a lot of drummers bought drums one at a time, or added another tom at later date. Usually it seemed that as long as the color matched that was all that mattered to them.
I've always thought that Slingerland and Gretsch lugs look similar and the sticksaver hoops even resemble the die casts on Gretsch drums. From any reasonable distance drums in the same wrap are going to look fine together. I've considered using Slingerland drums to add to or "complete" a Gretsch set although in both cases l found a matching Gretsch drum that filled the bill (at a steeper cost). The construction of the shells is different but, in the end, they all sound like drums. All of this is much less of an issue when the "odd" drum is the bass drum which isn't as visually prominent and is tuned much differently from the other drums.
All of which, I guess, is my long winded way of saying that seems like a cool set to me.