The older batteries will most likely have a higher impedance, thereby floating higher than the newer batteries. When this becomes enough out of balance, the newer batteries can float too low long enough to result in permanent damage. It is also possible for the older batteries to fail sooner than anticipated due to a much higher float voltage. It is most important to keep a log book on site containing all records of maintenance. This way, no matter who does the maintenance, all past readings and notes will be available.
Installation is generally the same from one manufacturer to another.
Using common sense along with the applicable information contained in the installation manual, you will be able to establish a set of guidelines that will work on virtually all brands of UPS batteries. In general, for a new installation or replacing a string of batteries, some of the important points include the use of the proper terminal grease, the proper connection torque and using new terminal hardware.