wow I really like your rack! Thanks for sharing.
I don't have answers to all your questions but here is what I do know..

1. After about 6-8 weeks your males and females can start re-producing. The litters are small but I've had a a 7 week old female become pregnant from one of her litter mates.. For males it's easy to see because their "balls" are visible after 6 weeks or so and they can reproduce no problem. Its the females you generally want bigger to produce larger and healthier litters.

2. Personally I wouldn't be comfortable with that gap but I don't have a lot of experience with racking systems, Rodents are very good at squeezing through narrow spaces. Hopefully someone with more experience can comment on this one.

5. I would say 100% that you can start breeding those two rats in the pictures. They look to be about 150grams + which is a good place to start IMO.

I'm just going off of the limited experience that I have so hopefully I have answered some questions, and all the information is correct. Good luck with your colony, and thanks for sharing with us!

Cheers.