To be honest I have 8" plants growing in a 3Gal next to a 3 foot plant in a 2Gal. All autos, same soil, same nutes, only difference is genetics. So I found it really puzzling on finding the right size container. Each strain is different so do what's best for yours. You can be safe and go with all 5gallons if you have the soil.
On the regulars, I'm very inexperienced so I can't be much help. I think there's an advantage to transplanting but I don't do it. I just start them where they'll finish it's easier lol. + my grow is in my bedroom so I'd rather skip the mess of transplanting. I don't know what I'm talking about here, just telling what I do. All the regulars I have I put in 4 gallon pots that I'm planning to keep till they finish. I think that's plenty of root space to grow for a tent-grow.