I didn't think platinum would tarnish. I know I will get a
truthful answer here. .
It won't tarnish unless, ghod forbid, the links were soldered shut
with low karat gold solder or silver solder. If they were, you've
not bought good chain...
As to shape, if you've got heavy enough links, and a style where a
little pull on the chain won't deform the links, then it should be
fine. Platinum can bend more easily than karat golds, especially
white golds. So if the pendant is too heavy for the chain, the links
may stretch out a bit. But they'll not break as easily as gold does,
nor will they wear down over time, as quickly as gold would do. The
main problem with platinum chain is that it's not as widely
available as are gold chains, so you may have to hunt around for the
style and weight that you wish.
Hope that helps.