Don't worry about hooking batteries in parallel.  It is common practice in
large diesel engine applications, and has been for the past 35 years that I
know of personally.  I have a farm, and the tractors all have batteries that
have been hooked in parallel to supply the necesary current to start the
diesel engines at cold temperatures.  Think about 9 to 12 liter engines in
the winter.  With a diesel, if the cranking speed is too slow, they just
don't fire.

It is good practice to start with a new set of batteries, but in the real
world I haven't seen a failure just replacing one at a time.

Chuck Hill