Just out of curiosity, what did you do wrong??? I changed the batteries in one without causing a fire (and I kind of expected it to explode because it had a questionable history). Where exactly did the fire start? I hope I don't cause a fire changing UPS batteries. BTW, was it plugged in when you changed them? Also, 7.5AH is slightly higher than the Smart 600 normally uses (mine had 7.0AH batteries in it).
I recently found out that the brand does matter when working with SLA batteries. I bought a "generic" 12V 12AH battery for my Back-UPS Pro 650 (it was made by Universal Battery) and after it had a chance to charge it started making noises! :Q It was making poping and gurgling noises. I took it out and they stopped after a few hours. My dad's friend (who knows a lot about cars, and thus SLA batteries) said it was overcharged. Luckly that battery was for a friend not me (it literally didn't even fit in his UPS. It was 3mm to big). My "Authentic" APC replacement battery never did that. From now on I'm only buying APC certified batteries!