Is a film holder absolutely necessary when scanning 35/120 negs? I bought a used V500 that didn't come with any accessories. I have found the power supply, but there is no neg holder. If it'll work just laying it on the glass I'm cool with that for now.
Don't know about the V500, but with the V700/750, the film holders hold the film in the plane of focus for the high-res optics. (At least, that's the theory.) So, yes, you absolutely do need them if you want decent scans. I would recommend a film holder from betterscanning.com.
On the V750, and probably on the V500, you can set the Epson Scan software for using a film holder, or not using the holder (I forget exactly what that is called). I've scanned odd sizes of negatives on my M750 and they are sharp. But beware Newton Rings! I tend to get the best results by placing the negative emulsion down against the glass, and flipping the image in PhotoShop.
That's true, but the high-resolution lens is not engaged when placing the film directly on the platen (don't know whether this is true of the V500). And, as you rightly point out, Newton rings are almost a certainty.
With any decent Epson scanner, a betterscanning holder is, IMO, pretty much a necessity, so may as well buy one at the start.