
Rodimus looks a bit top heavy to me, but Springer is by far my favorite

As for groupings, you could go w/ character groups like you have, but you might also wish to consider grouping by characters that are officially available too. For example wave 1 has a Springer so folks are going to want a new helmet for him. Same w/ others in that wave that you may have done but aren't shown here.
Happy to see you've dialed in on the exact measurement to go with for the inner diameter, thank you for sharing it

I think that the helmets are large enough that you can get away with having them just close to each other without having to put them on a sprue. We do that mainly when there are small parts that we want to keep from getting lost. These should be safe
