I'd be a bit nervous about putting batteries in parallel in case for some unknown reason they discharge at different rates - does that even make sense / is that even possible though in this configuration? After all you did say to equalise them to begin with.
Yes, this is (or at least tends to be) a serious problem.
We have a share in a narrowboat, where the electrical supply (when not plugged into a shoreline) comes from a bank of 4 x 110Ah batteries, wired in parallel, in theory giving us access to 50-150Ah for overnight use.
The bank of batteries is one of its weak spots, even after a lot of thoughtful attention. Invariably, the bank isn't balanced, and one of the batteries gives up the ghost far too soon. We can't get them to last more than a couple of years. After that use, you need to replace all the batteries in the bank at the same time, so not a cheap failure.
This page gives some information about wiring the bank to achieve better balance:http://www.smartgauge.co.uk/batt_con.html
That page highlights the different current that can come out of each battery (or goes into it when charging) when badly wired. All down to the small internal resistance of the batteries and the small resistance of the cables. An imbalance in the resistance means that, while the voltage is the same, the current is not.
The right wiring ends up like this:http://erin-mae.blogspot.co.uk/2016/05/battery-balancing.html
Parallel batteries have to stay at the same voltage (since they're in parallel...), so it's safe to put in parallel any batteries of the same chemistry even if they're of different capacities.
If they are going to actually have significant current in and out, then it will pay for the batteries to be as identical as you can make them. Variation in resistances means variation in current, means some batteries will get degraded lifetimes.
As you say, though - UPS batteries tend to sit in a fully-charged state, with a float current running through them. They aren't likely to degrade much in this condition - only when they are being used.