Rust on chain and cassette
It's unclear how much rust you're talking about, but it's unlikely that a bike could develop serious rust on a chain in 5 months unless left outside with no maintenance.

I've dealt with some pretty rusty bikes over the years (especially when I spent a spell working on donated bikes for Christmas Anonymous) and I've never seen a cog that was so rusty that it was unserviceable.

The chain, of course, is a different matter, but generally rust on a chain is of little consequence unless it causes "stiff links", and that won't happen on a bike that's used regularly.

I'd recommend that you switch to a slightly less "dry" lube (or at least give the chain a few squirts of regular oil every now and again).  Also, if you've been cleaning the chain at all, be a little less diligent cleaning it -- it's not necessary to get it shiny bright, you just want to get off the crud.

Otherwise don't worry about it.