In fencing applications, we recommend the 1 inch long (2.5 cm) u-nails from Devki Steel. Pre-drilling is not required as the u-nail will easily be driven into the plastic post using a hammer.
If you are using screws, pre-drilling will be ideal though not essential. Remember not to tighten the screw down to allow for expansion or contraction.
On average plastic poles hold nails and screws better than wood.