Hi HJ,
1. 1" at 4 weeks is about right. Good job getting them to this point from that size.
2. I think most folks would say about 1 year, but some possibly a bit sooner, and adolescents will definitely start showing some behavior, although often nothing comes of it. As you noted, males are usually considered to take longer to mature, or to really do their thing when the time comes.
3. I'd say this is a possibility, which puts you in that typical spot where you won't know until they spawn and fry actually hatch.
I'd also add, that as a likely pigeon blood based red, the fry from a cross with your others is likely to be peppered and not real attractive. It can still be fun to go through the experience, but if they were successful, you might end up with fry you don't know what to do with.
Your fish do look good, though!
Steve