To be sure it's OK:
- Water them all to the proper height.
- Do an equalizing charge (to make sure they're all equalized). Rewater afterward if necessary.
- After a bit of service (a couple days) to use up the surface charge and get the added water mixed, use a GOOD hydrometer to check their state of charge (on all cells) while fully charged, and again while partially discharged.
If the cells of each battery are all about equally charged you don't have a bum cell or an equalization problem. If a battery has unbalanced cells a couple days after equalization it's probably defective: Pull it out before it kills its partners. (Then let the others come to full charge again after the bummer is gone.)
If the state of charge also matches pretty well between the differing batteries, both at full charge and at partial discharge, you're OK. If some show a higher charge than others you've got a problem: Some of the batteries will keep the others from being fully charged, leading to sulfating of the charge-later batteries.
Or at least that's how I understand it. (I haven't had to maintain a big battery bank myself...)
