-a bitcoinAddress
do I need to add anything to the bitcoin.conf?
Not to change the node fee payment address. -a sets that address, as jedimstr said above.
So I don't need the disablewallet=1 in bitcoin.conf correct, only need to add the -a {address} to my P2Pool startup string?
No, you don't "need" it... but I use disablewallet=1 anyway since I only use bitcoin-qt (or bitcoin core since that's what we're calling it these days) for it's blockchain RPC and not as a wallet.
Note: disablewallet=1 only works on bitcoind/bitcoinqt OMG10 and 0.9.0 rc2 at the moment (that I know of).