How hard is it to form neck down the 308 brass to 260 Rem? Do I just FL size it with my 260 dies? Turn necks? Ream ID of necks?

I haven't stopped at .260 when forming but I've gone down to .243 from .308. In some instances you can make it with one pass into the FL .243 die, but depending on the brass, you might need an intermediate step. I used a 7-08 die as the intermediate step and had no problems with it. I don't think there is enough difference in the 7-08 and 260 to matter. I've had some that formed easily in one step to 243 and I've collapsed a few shoulders trying to do it with other brass. Try it.

There is amazingly little thickening of the necks. I've never had to turn one coming down from 308 to 243 as the difference is usually less than .001". Lube up your neck and try it. Measure the neck of a case fired in your rifle and once you've sized on down to .260. seat a bullet and measure the neck again. If the difference is at least .002", load them up and just go out and have some fun shooting.

I think this will depend on the brass used and the size of your rifle chamber. I had a big pile of 308 Lapua brass and no 7-08 brass so when I bought my Galil 7-08 semi-auto I thought the logical (cheap) solution would be to just size down the brass. Unfortunately the cartridges would not even chamber in the (thight) chamber of the Galil. Had to buy a Forster neck reamer and remove a fair amount of brass. Very laborous - ended up buying ready made 7-08. Maybe some other brass (Rem?) are thinner.
Rgds.,
Peter