All the Roman codes seem to come with oval shields (pictures below) which I will mention more of below in comparison to the Hat figures. Check out the webstore!
A few comparison shots now, there's a clear sculpting style difference between Hat's offerings even, so if that can be accepted then the Newline style shouldn't be an issue. Size wise they are all usable with each other being generally the same height. I'll end up using them within the same army but have same sculpting style on a single base which is my normal preference.
Where there is considerable difference seems to be in the shield size. The Newline oval Roman shields are smaller than the cast on shields on the Hat legionaries. The larger shields seem to be more in keeping with other examples of Late Roman miniatures and the Armies and Enemies of Imperial Rome which features quite large looking shields for the late era, one source suggested about 3.5-4ft long 3ft wide (roughly 17mm x 12.5mm in 1/72) while the Newline one is 14mm x 10mm. This seems more in keeping with a some of the inspirational osprey plates I've come across. Cavalry shields on the hat figures are roughly the same size these will mix well. When it comes to the Goth's shields I have even less of a clue but I don't have any plastic infantry to compare anyway. They measure at 10mm and give the option to give Romans circular shields if you prefer, which might give them a more post Rome look.
Essentially it's the customers choice whether your units have uniform or mixed sizes but when it come to Hat they are cast on and would take a considerably effort to change. Newline on the other hand with their separate shields gives you the freedom to swap them to match the cast on shields of the plastic Hat figures depending on what look you are after. I'm looking forward to get some bases done of regular Roman and post Roman units as well as some enemies!