Can I use PT950/Ru or PT950/Co stock to resize this ring? What
kind of problems could I encounter if I attempt to do this?
Yes. You can use the other alloys. There can be a very slight
difference in color, but you'd have to look very closely to see it,
and even then, maybe not.
If you do the sizing by using platinum solder to solder in a piece,
then the alternative alloys won't matter so long as the solder is not
too high a melting point for the alloy you're using. Some of the
alternative alloys melt slightly lower than iridium platinum (pt900
is most likely iridium/platinum). If you use solder, I strongly
recommend the "plumb" platinum solders rather than traditional types,
which have less platinum in them. The traditional types, unless you
use the highest melting grades, produce seams that can polish out, or
show a difference in color. The plumb solders are invisible after
soldering and cleaning up the joints.
If you use either a torch or laser to weld/fuse the seams, then any
difference in the melting points may require slightly more skill to
get a properly done seam without areas at corners/edges getting
melted in, forming depressions you then have to fill in. With a
laser, it's a minor problem. torch fusing can be a little trickier.
But you can do it. Between Ru or Co alloys, I'd choose the Ru alloys