I don't think that it will spoil your battery - you'd just have longer charging times with your new battery. Also the new MBPs come with a MagSafe 2 adapter. I don't know which adapter you have but you'd need to purchase an adapter for your old MagSafe charger.
If I were you and needed more than one adapter, I'd just bite the bullet and buy a new MagSafe 2 adapter from Apple that is guaranteed to work with your battery and computer.
You will not damage your computer or battery using the 60W adaptor. Depending on what you're doing, it simply won't be able to provide enough power to both charge the battery & power the computer. While your computer is off or sleeping, it will charge but slowly. The most that can happen is the adapter fails from heat, after all it is being used for something it was not designed for. Despite my suggestion not to do this, my daughter in law does this regularly & has been doing so for the past 6 months...she got an Air for Christmas & also owns a 15" Pro. If there were any chance of damaging your computer, Apple would design these things so that they could not be interchanged.
i just went to bought an adaptor and it worked!
so to all people facing the same dilemma here, just go get the adapter. charging might be abit slow (apple staff and the 'helpers' here).
but the apple staff told me that 85W shouldnt be used on MBA. contradicting some of the things that i saw online.