Good article – and in fact, we did the same and promoted our UX lead into our first product manager. Just two points I’d make;

• We needed a product manager before we reached 15 devs. I’d say the idea size is around 8 before you need your first full time PM (although that depends on the quality of your devs and product!)
• You need to account for the lack of experience with your first PM if you’re promoting internally, something I feel we’ve suffered through. I’d be interested in hearing about how other startups have managed to fill that gap.