This is purely out of curiosity. I have had quite a few pre-G3 PCI Macs over the years and they all seem to boot up System 7.5.5 despite Apple saying that some later models require 7.6.x. The 8600/300 I have boots 7.5.5 without enablers which, according to the lists below, should be impossible. This era of machines was a bit weird since there were Mac clones out on the market. Once the G3s came out in 1997, Apple jumped to 8.0 finally to exploit a loophole in the clone contracts.

http://support.apple.com/kb/TA36684

http://support.apple.com/kb/TA47341

Going by that, I think the only machines that might require 7.6.x as a minimum are the Powerbook 2400 (oddly missing from the list) and 3400. The 1400 shipped with 7.5.5 + Enabler. Having never owned either machine, I can't confirm that. I'll assume the PB3500/Kanga G3 required 8.0 like the desktops.