Issues with changing to compact chainset

[Looseendorg]

Hi,

I've just changed my 9speed mirage set up from 53/39 to 50/34 and at the same time changed the cassette from 12/25 to 13/28

It shifts ok, but in the second to largest sprocket at the back the derrailleur cage is rubbing against the largest sprocket. I let the B adjustment screw out as much as I could but to no avail. Further research shows that the short cage derailleur can't handle the difference in tooth count (chain wrap?).

I'm hoping I can get a medium cage mech to fix the problem but have a couple of questions and wondered if anyone on here can help?

* will a 10 speed derrailleur work ok with the 9 speed setup? I assume so as the width of the cassette is the same.
* I put a new chain in when I changed all this, will I need to lengthen it to cope with a medium cage, or will I get away without doing so?

Thanks in advance for any help anyone can give.

John