My CO has a pack, troop and crew that are operating with an American Heritage Girls troop as a sister unit. Finances for all units are separate, but operate under the same CO at the same time. There is a lot of crossover for both programs. We hold meetings separately, but join together for events and service projects. My girls are also part of coed BSA programs, one being a pilot program based on Venturing but for middle school ages (which I am associate advisor for). We have a lot of interest in my district for trying something similar for Cub ages. My pack also started up the first Lions den in my council because we need something for kinder boys. We were just going to offer our our own church program alongside the Cubs- and eventually garnered council support.
As far as YPT concerns in a coed group, the BSA has already tackled that. Match regulations for Venturing- a leader of each gender for every time 2 deep leadership is required, and buddy groups of three in a mixed gender group or of 2 in a same gender group.
As for insurance- let's don't be silly. FG is likely to have their own type of coverage. If not, then the CO is (like in a church- where offering different ministries to boys and girls at the same time is nothing new.)
My advice as a program planner across different platforms: find where the programs have parallels. Where policies don't match up, go with the most stringent policy.
Congratulations for thinking outside of the box and trying something new. Don't be daunted by the naysayers!