My house bank is six 220 amphour 6 volt golf carts wired into a 12 volt
system.

I have a perfect place for another battery, but two golf carts won't
fit there. I can however, fit a good size 12 volt deep cycle there.

Do you see any harm in wiring a 12 volt battery into the system?

In other words, mixing the 2 x 6 =12 volts with a 12 volt.

Thanks,

Mike.