I have a pair of open fronts (that are horse sizes and I ride a pony with tiny legs...this was when I had my mare).
Polos probaly won't give the support you need (and if you EVER try to use them, make SURE you know how to put them on correctly).
You can never be safer then sorry with jumping, that's why I suggest boots on all horses; no matter the "shape" (of coruse, for example), size, or level of jumping.
Although it reminds me that I need to buy the pony-kins some boots.