You can definately make 7/08 from 308 and 25/06 from 30/06, that's what the original wildcatters that invented those cartridges started with. And if you have a supply of quality, cheap brass (especially 308 military) that is readily available, it makes pretty good sense to me.
I've made a lot of cases from other cases, mostly because I had a lot of one type, needed another type and had lots of time on my hands.
A couple of years ago, I bought a Yugo M48 Mauser 8X57mm and needed brass. I had a LOT of 30/06 brass (10k+) so I spent a few days forming some 8mms from 06s. It can get a little tedious, but I enjoy the process of reloading and tinkering.
No matter what you form or what you for it from, please just to be safe, always check the finished case length and inside/outside neck diameter.
W101