I'd oppose against Starmie if Rapid Spinning is the only reason of using it. With the high chance of Rotom forms appearing on stall teams, there's not much use for Starmie and Rapid Spinning in general. However if you want to use a LO set that can 2hko the standard Rotom then by all means try it, just not positive on having Rapid Spin on that same LO set, negating the whole purpose of Starmie in the first place. However, ToF is right about Spikes and Toxic Spikes causing trouble, so Starmie is something to try against those rare stall teams without Rotom or hell any Ghost.
That said, if you're going to use Starmie, then I'd highly recommend CB Tar over Hippo so you can pursuit Rotoms and Dusknoirs and open up a BP sweep for Scizor or prevent them from WoWing your Rhyperior. It's simply a good wall breaker in general, especially when faster than Skarmory (96 Spe EVs to outspeed Skarms with Spe EVs, may seem like overkill but it's worth wild imo). However I guess WoW can hinder the concept, at least in conserving HP. That also said maybe fit in Wish or even Cleric Bliss? Possibly over S-Toss and add Flamethrower over Ice Beam? Now these are changes if you were to add Starmie, but with T-Tar added your now fucked by Lucario even more, and in addition DD Mence (ugh me and my terrible suggestions, my theorymon seems good but overall produces crappy results, oh well best I can do lol). Still Toxic Spikes is the main concern as it's the reason why Hippo will fall to Lucario and Salamence in time.
Nitpicking... keep HP Electric on Slowbro, Slowbro with Surf beats Swampert anyway, at least non Curse versions. Besides, HP Grass; T-Bolt; Heat Wave is good coverage for your Zapdos there in dealing with Swampert.
Overall, you have the common threats down. Nice team Hector <3