I have an old APC 1500VA UPS, by old I mean I bought it when Circuit City went out of business.  It kept failing self tests a few years ago so I put it in the garage and forgot about it.  Recently opened it up and one of the batteries is bulged.  A bit of research says it uses two 12V, 9AH batteries.

I have access to plenty of 12V, 7AH batteries that we can't sell due to being outside internal specs as far as age is concerned.  Brought 2 of them home and one is at 13.02V and the other is at 10.35V.  The 10.35 is on a charger right now and if it won't stay above 12 for a day or two after charging I'll get another one.  "New batteries" are sealed lead acid.

Question is, can I use the 7AH batteries in the UPS?  It will only be powering a Synology NAS and maybe a 5 port dumb switch, or for power outages it will be a phone/tablet charger.  Not really worried about the power capacity, more worried about causing a fire.

I've always related amp hours to run time, so I don't see an issue using a lower capacity run time battery.

At a previous workplace if we seen batteries putting out 13+ volts we'd retire them.

Just lower run time gtg

Yup it will be fine.

No problem. They're also close enough in AH rating the internal charger won't do any damage to them.