I wouldn't use polos. Polos keep the legs just as hot, don't have a lot of impact absorption, and are extremely time consuming and easy to mess up --- And this is coming from me, who's been a polo nazi and supporter on the forum for ages.
Plus, if you're on the trails, SMBs and Polos are going to get stuff (Foxtails, burrs, prickly plants, leaves, stuff like that) stuck on them
Here's what I would do. I would get some ProTech Splints/Skids, for front and hind.
They have the hard plastic on the side and back, and are super fast, easy, and they wont make her as hot as the SMBs (Most everything is going to have her sweating at least a little, but these are better)
Also, what bell boots are you using? I really dislike rubber bells....I mean, rubber would be good for hte pasture, but for rides I like something harder. I use Classic Equine No-Turn DLs. Best things ever.
ProTechs: Classic Equine Pro-Tech Boots - The Tack Stop
DL Bells: No Turn DL Bell Boot - The Tack Stop
And, because she's cute, here's a picture of Selena wearing her ProTechs and DL's. :)