You have it backwards about the 7.62x51 and 308win. The 308win can fire 7.62x51 ammunition, but the 308win is too high pressured to fire in older 7.62x51mm rifles. Certain battle rifles tuned for 7.62x51 (such as the M-14/M-1A and AR-10) can experience excessive wear and even possible failure if you use .308win loads.

On the mosin nagant, personally, no, it's not worth the money to me. Some of them can be exceptionally accurate, and I suppose if you're planning to leave it in it's original, unaltered state, then it's a good deal. However, if you plan to buy the mosin Nagant and sporterize it, then you are wasting your money. By the time you buy the rifle ($100-130), change the stock stock ($75-100), add scope mounts ($100+/-), have the bolt cut or bent ($60-100), etc etc you've already spent enough money to buy a decent modern rifle. Then add the on-going slightly higher ammo cost (for decent hunting grade ammo, of course surplus ammo is dirt cheap), and you've wasted a ton of money on a rifle that's STILL only worth $130 if you'd ever resell it (yup, spending $400 sporterizing it DOES NOT increase resale value).