A bad lifter can ruin a camshaft quick, not to mention spreading metal fragments though an otherwise good engine. From the sounds of it, the lifters you have are junk. I wouldn't risk just replacing one or two, as it could cost you much more in the long run (like having to change the cam and lifters anyway, and premature wear on the cylinders/bearings, etc). Replace all 16. A set of lifters costs less than $100. Much cheaper than rebuilding a whole engine.

You can use new lifters with an older camshaft, just not the other way around. Treat it as if you were installing a new camshaft. Put in fresh oil with Zinc additive, thoroughly lube the camshaft lobes and lifters (soak the lifter in oil first) as if you were doing a regular camshaft install. Do the break-in. Then change the oil again and use a zinc additive again.