Well, it seems as if you have already researched everything there is to know.
- Originally the SE/30 was introduced with 6.0.3
- Official support for the SE/30 goes until 7.5.5
- Since 7.6(.1) it is still just a minor update. It can be easily installed.
- System 8.0/8.1 does work as well, by copying a few files from 7.5 (*1)
The choice which System to use is mainly defined by the machine configuration.
- Without a hard drive only 6.0.7 makes any sense
- Similar with less than 4 MiB RAM, as 6.0.7 only needs like 3-500 KiB RAM, leaving acceptable amounts RAM for an application
- With 4 MiB or more System 7.1 may be the best choice, as most software will run under 7.1, while it is still somewhat lean (*2)
- With 8 MiB or more the newer systems may be acceptable (*3).
But there are also software issues, as System 7.5 introduced a lot of new and quite handy improvements (like a scripting ability for Finder operations). 8.x added even more nice stuff, like multithreaded Finder (background copy) or the neat platinum UI. On the back side, everything after 7.5 does need clean 32 bit ROMs. Further, 8.1 is the last system supporting 68k Macs (*4).
The ROM issue is a bit more complex - and tied to RAM size as well. The Mac ROM includes large parts of the basic system code, thus relieving the need of RAM to load this code from disk. Originally no-one envisioned a Mac with more than 24 bit addressing (*5) like the original 68000 offered. Thus many routines either didn't really take care of the uppermost 8 bits, or worse, used the top 8 bit in every 32 bit word for some bookkeeping. Such code was present in the ROM code and not fully removed until the early 1990s.
All SE/30 are equipped with such ROMs, originally preventing the use of more than 8 MiB RAM. Since the machine can be fitted with up to 128 MiB (Awesome for an 68k Mac) Apple had to come up with a solution to satisfy complaining customers. Instead of supplying them with clean ROMs they bought a MODE32 called third party utility, offering a 32 Bit layer around 24 Bit functions. Of course nothing comes with a backlash, in this case, a few programs relied on these functions operating as they did before - leading to crashes:(
So while 7.5 runs fine with MODE32, 8.x directly checks the ROM ID and refuses to work. There are ways to make 8.x boot with MODE32, except they are not really legal. The only legal way is acquiring a clean ROM. Lucky here the Mac II (IIsi, IIfx, etc.) family machines use the same ROM module format. So acquiring one from eBay (or any scraper in your neighborhood) will do the trick. Except no it'll fail in some programs compiled for an 68040. This can for most parts be resolved by swapping in 7.6 files, but it might be a rather large effort even for people custom to this.
*1 - Plus clean ROMs or an emulation thereof.
*2 - If you got more then 8 MiB and the Finder tells you that the System needs like 12 MiB or alike, than you might want to install/activate 32 Bit addressing - as without only the first 8 will be used. The larger number displayed for System is due the fact, that everything that can't be accounted for otherwise is declared as 'system :))
*3 - Of course there is always a chance to try to slim it down to what is needed - still, it wouldn't change the general notation.
*4 - It is said that with a lot of extensions hacking it should still be possible to run 8.5 on a 68k system.
*5 - In reality only 23 Bit (8 MiB) as the top bit was used for management.