Li-Ion and Li-Poly in parallell are just fine. The important thing is that they are both same voltage when you connect them together, after that they will stay balanced. Why? Because in lithiun-ion (including lipo) chemistry, the voltage has direct relationship with state of charge, so if you draw 1A from a small battery, its voltage drops faster than the big battery. When the voltage gets even slightly out of balance, the bigger battery with the higher voltage supplies more power. The end result is that both batteries will have become full at the same time when charging, and empty at the same time when.discharging. Some smartphones and tablets these days use "stacked" batteries, several batteries with different shape and size to fill up as much empty space as possible.