I'd have to look at some other notes, but you're right, these are all largely the same platform, with various features added and removed per the needs of the product Apple was trying to create at the moment.
Off hand, the one thing to be aware of is that while in theory any of these boards support the same speeds, I've seen it reported that stock cache modules on the 7500/100 in particular tend to be the "best" at varying speeds, because the 7500/100 in particular shipped with the bus set to 50MHz, as the best match to its 100MHz CPU. Most of the other models shipped at ~40-45MHz.
(A point to double-check: what about the /200 models?)
The other way to work around, say, a cache that works at 40 but refuses to work at 50, is to get a different one.
The other main differences:
7500/7600: AV input but not output (main imagined use case seems to be knowledge workers/executives who wanted to do video chatting, but it should work fine for a/v authoring, minus dumping back to tape)
8500/8600: a/v input and output
9500/9600 (and most 6-slot clones): up to 1.5gb of ram and six pci slots, no onboard graphics
7300: same as 7500/7600 but no a/v.
7200/8200: This is probably not within your interest realm because these shipped at soldered CPUs at fixed speeds, but they're a similar overall platform but with a lower RAM ceiling and of course the soldered, rather than slotted, CPU.
This is more on the "historical conjecture" and "product stack planning" but the 7300 is usually framed as a weirdly numbered successor to the 7500/7600, except:
In APAC markets, a 7600/200 existed.
Personal theory:
The 1995 lineup was (excluding the low end)
7200
7500
8500
9500
upon introduction in ~1997, the "new" lineup was:
7300
7600
8600
9600
But in reality Apple was TERRIBLE at numbering and managing the product stack and the hot reality is that the 9500 and the Mac LC 520 were shipping at the same time and the 7200/120 and 6360/160 stayed on sale alongside the "middle" group of PCI PowerMacs (7300-8600-9600/200 in particular) (This is all down to Apple thinking they had a good Capitalism Excuse™ to put more than ten nominally unique product lines on sale, so they overloaded 6 (6360/6400) and 7 (7200/7500) into two each unique product stacks. They did the same thing within the 5-series, the 5260/5280 stayed on sale as a budget-conscious/EduCheap version of the AIO for a fairly long time along the 5400 series, and then a low end 5400 loadout itself stayed on sale below the 5500.) (ultra-ironically, and I know this is getting incredibly into the weeds, but it turned out keeping something like 37 unique models and stock configurations on hand at any given moemnt was absolutely insane bonkers expensive. Once Apple replaced *every* other model with a G3, resulting in basically five products all built with the same parts, they were able to cut the price of every single mac by minimum a full thousand dollars -- the 7300/180 had been $2300, the G3@233 slotted in at that exact same price, in 1998 they discontinued the powermac/233 and re-priced the powermac/266 desktop at 1299, every model above that got the same or similar cut.)
You probably already know this but the 8600/300 and 9600/300-350 are based on a revised version of the architecture, usually suffixed as Mach5/MachV - these put the 1-meg of L2 cache on the CPU card and have a few other differences, all variants run the bus at 50MHz, these are a little more strict about software (shipped with and on-paper require 7.6.1 and an enabler, but these are fast enough to run 9.1 competently IME, so what you choose will basically be down to vibes and needs).
On paper, G3/G4 upgrades and the Apple (and others but I forget which brand) faster 604 upgrades work with any of these machines.
But, whether or not that's helpful really depends on what "overclocking" means to you. If your'e just looking to run the 8500/120 (40mhz bus) at 50MHz, that should work great. I'm actually running a 601@100 in my 8500. If you're looking to try to get the bus to be faster than 50MHz, I'll defer to anyone who has actually done that. For various reasons, I've never bothered.
Hopefully that's helpful // good luck! (definitely put any followup questions in, I or someone who has more bare-metal knowledge than me will be able to help!) (also my apologies for the wall of text and so many sidenotes!)