As only block size is being increased, wont bitcoin core coins be compatible to bitcoin XT as their block size is lower than the proposed one for bitcoin XT ? Just wondering 
If you keep running 0.10.2 then yes, but if Gavin leaves the Core team and the Core changes to include side chains then those versions of Bitcoin Core will not be compatible with Bitcoin XT and a full fork will occur, which is why I say make a backup of your wallet NOW. It's also more important than ever to check what changes are being made to the client you're using so you know whether or not they cause a fork.What about those who are not using QT/XT, i.e. using blockchain.info, electrum, armory etc. ? What would happen to the coins which are kept on paper wallet and p2sh addresses ?