I've noticed that many centaur item slots (back, waist, tail, hooves, legs) are unique and normal gear cannot be worn in them

That is by design. Just like tail items previously could only be worn by characters with tails (Felar), now they can only be worn by characters with tails (Felar and Centaurs). Just like hoof items could only be worn by characters with hooves (Minotaurs), now they can only be worn by characters with hooves (Minotaurs and Centaurs). Centaurs have equine legs; there is a reason you don't see horses wearing pairs of pants.

I've noticed that centaurs are unable to wear about gear in their back slots.

They wear gear designed for their (equine) backs.

I think it would be a good idea to let centaurs wear normal legwear... and about gear...

With the unique anatomy of the centaur, I'm more interested in demonstrating racial differentiation. We did this to a lesser degree with Arials, Felar, and Minotaurs--I'm doing it to a greater degree with Centaurs.

I tried a centaur for a bit and noticed that fine leather...

When Centaurs were announced, we posted that they could not currently complete the fine leather quest.