Lithium Ion and Lithium Polymer are the same kind of batteries with small differences in chemistry and internal construction.

Both types are interchangable and allowed. The only thing you have to watch out for is discharge rate. Some Lithium based batteries have a rather low charge/discharge rate, called the C value.
2000mAh 10C can deliver 20A for 6 minutes, but 20 amperes is low for a featherweight, and there are beetles that demand more.
Demand more from that battery, and you will damage it, and possibly set it on fire.
The LiPo(or Ion) battery should (according the rules) be protected with a fuse, rated at the maximal capacity of that battery, just to avoid this.