Some thoughts:
There is no connection between hard drive size and the ROM, original or replacement. The size of the hard drive is controlled by the System software. E.g., System 7.1 will work with a drive up to 2GB in size; after that, partitioning is necessary under 7.1. I think System 6.0.8 is the same.
You would not really want to go beyond System 7.1 in an SE/30, at least not if you want it to run fast. System 7.5 is much more cumbersome. The machine was really designed for System 6/ very early System 7. Note that System 6, 7.0 or even 7.1 will find that 8MB of RAM offers it plenty of room for manoeuvre — especially 6.
A 500 MB drive will frankly be hard to fill up in an ordinary SE/30, and IBM made some for Apple that were real nice and quiet, so I would go for one of those; even a 2GB drive would be almost ludicrously large in an SE/30. There are nutters who stick 9GB drives in the little machine, but that is a waste of a 9GB drive — and it has to put strains on its electrical system, since the faster that thing spins.... For most uses, 500MB is plenty, and assuming you have one to spare, just use it.
Replacement ROMs in an SE/30 have to do with the computer's ability to access large amounts of RAM — but that can also be done by a software patch called Mode32, which is readily and freely available on the web, so I would not go about ruining a IIfx just to get its ROM for an SE/30. However, if you happen to have, say, 16MB+ of RAM, I would forget about the dirty ROMS issue, and look instead at one of the old RAM disk utilities (e.g., Maxima) that allow you to use RAM above the 8MB maximum as an ultra-fast RAM disk. RAM compression technology in Maxima allows you to double or treble that. Those little tricks are, in my view, more interesting to work with than having 16MB+ of RAM to play with in early systems like the SE/30. 8MB was a LOT of RAM back when the machine was manufactured.
The one OS that does need plenty of RAM in an SE/30 is A/UX — but it will recognize up to 128MB of RAM without further ado. Not sure why. It would also be happier with a drive up to 2GB in size, as it is a hefty installation. It runs, but runs rather slow on an SE/30.
Returning to the drive, you would also find your path much easier if you were to choose an Apple-branded 50-pin SCSI drive (they have the Apple logo on them), as the standard System utilities will not initialize third party drives (though the A/UX disk utility will). And there are other workarounds if need be, but we'd need not go there just yet.