Aside from the difference between 7-speed and 8/9-speed freehub bodies, are Shimano freehub bodies interchangeable or will I need to replace model-specific parts?

The specific hub that I'm screwing around with now is the 7-speed RSX FH-A410 with 130mm O.L.D. but I would like to know the answer to this general.

I'm no stranger to frankenbike building. Could I even take an 8/9-speed freehub body, an axle meant for a 135mm O.L.D. hub and some axle spacers to turn this into a 8/9-speed hub?

Something isn't right with this wheel that I've been given. I've cleaned and repacked the bearings to find that the cones will either be too loose or too tight. From what I can see, the non-drive side is fine but something is wrong on the freehub side. Either the inside of the freehub body or the opposing cone is worn out in some manner and the ball bearings aren't being properly held in-between as these should.

I do that all the time. We get old 7 speed freehub Shimano wheels in all the time at the co-op I work at and swap out the 7 speed bodies with 8-9-10 freehub bodies from trashed mountain bike wheels all the time. I space them out properly too to either 130mm or 132.5mm. 132.5mm is nice because then you can easily run that wheel in either 130mm or 135mm frames.

Only thing you have to do after swapping out the components is check the dish and redish the wheels.

Yes and no.
I've swapped older ones.
I tried to swap a new RM-30 body to M-570 hub and it wouldn't work.

Do you possibly have an incorrect cone in the hub?

I have no idea. All I can say is that I didn't put different cones in. I pulled out the axle, cleaned everything as well as I could, put in fresh marine grease and put this same axle, with the same cones and all, back in as well as I could.

I performed essentially this same procedure on the front wheel of this wheelset and this worked out very well. I've also repacked other rear hubs successfully before so repacking this wouldn't be a first time for me.

Proper number & size bearings?

I've come across one or two cassette bodies that were a bit wonky like that, either in the bearing cup or actually at the spline, I suspect.

I pulled a sneaky on my friend's Diamondback road bike. I had a set of 8sp Sora brifters kicking about that were some hundreds of dollars cheaper than buying new and somewhat more user-friendly than his old downtube shifters.
Unscrew or drill the rivets out of an 8sp cassette body, add a thin spacer first 1 or 2mm perhaps, then 7 cogs of your choice with the correct spacers salvaged from the cassette and then your lockring. Set the high and low limit screws accordingly and nobody would know any different. Scrub the "8sp" label off the right-hand shifter if it bothers you.

Remember to also fit an 8sp chain!

The benefits of this are great.
1. You get to use readily available brifters on an old system
2. Future proofed for a new wheelset. Upgrading will let him have his 8 speeds as desired.
3. Cost-effective!

The "gotcha" that I've run into when swapping cassette bodies around is that the "mountain" hub shells have a slight recess for the cassette body where the "road" hubs do not- the right surface of the hub shell is flat (ish).

I had this bite me when the 8-speed cassette body I had combined with an old Deore XT shell failed while on tour in Canada. The tour's mechanic was able to get a replacement body at the next town (he loaned me a wheel so I could finish the day), but it was a "road" cassette body. It goofed up the indexing a bit, and the rear derailleur cage "tinked" on the spokes in low gear. No huge problem, but I avoided using the big cog for the rest of the tour.

This may not be the case with all Shimano hubs, but it is something to look out for.

There is incompatibility between many of the shimano freehub bodies and hubshells, I couln't name them all.

XT didn't work on LX, and 7700 didn't work on a 6410 Ultegra hubshell.

In both cases, the sprockets ended up the wrong distance from the driveside flange. and thus from the spokes.
The wheel dish was thus altered, not for the better.

Moral of story is to make no assumptions about any freehub body working properly on a different model of hubshell.

There are also the better-known issues of one freehub's cone/seal/spacer/locknut not working with a different hub's freehub body.

I lucked out. I haven't been able to find any spec sheet for the FH-A410 BUT Shimano does have a freehub interchangeability chart within which this listed.

The part # is Y3AP98020 which is also used in the FH-M290, FH-M330 and FH-MC32. I can get the part for about $17.

Should I push my luck and try to find some kind of a 8/9-speed version of this or just get this and be done with it?

I forgot about having the wheel this is on until I read that the Mavic MA rims that are on this wheelset have the same ERD as the Sun CR18 700c. I could repurpose these wheels for my beater bike and maybe also use these for touring.

You can find complete RM-30's online for about $20.

Maybe. Would I be able to reuse the spokes?

Shouldn't be a problem. Flange diameters are the same and the offset difference is insignificant.

Just an update on this wheel: I took a second look at the hub and found that I had placed backwards a part that slips on the axle on the freehub side.

This seems to be a part that acts as a spacer and helps holds the rubber seal onto the side of the freehub where the ball bearings are.

Working under a fluorescent light during the winter, I didn't see that I had placed some kind of metal ring that's part of this spacer, instead of the rubber seal, was making contact against the freehub edge. Working outdoors, in the daylight, I was quickly able to spot my mistake. Oh well...

As I was looking at this "spacer", for lack of a better term, I could see that if I were to lose this piece then I may as well throw the hub away.