6 volt golf cart batteries are deep cycle batteries. While there is no harm in using them to start a motor, they won't have as any CCA as a dedicated starting battery. They use thick plates to hold more energy but they can't release it as fast.
Starting batteries (including most dual purpose) use more thin plates. The extra surface area allows them to discharge more quickly (ie: more CCA). Great for starting a big diesel but they don't hold as much energy and they won't hold up as well to deep cycling (ie: they will have a short life).
As someone else mentioned, generally a bad idea to add a new battery with an old battery. The old weaker battery will constantly be trying to pull the new battery down to it's level.
