JP Fan Guide contains accessory Cost list with the Episode 6 Part 6 NGS Engine Update.
Also found out through this fan tool that Sgt. Frog (Currently streaming on Funimation) had a JP collaboration item.
Thanks to this tool, I learned that I indeed have to remove the Leg Wheels in order to add both Shoulder Arms.
I'll just hope that Cygames agrees to an official SEGA PSO2 NGS collaboration deal so I can get some Finni CAST Legs.
(Or wait and see the new NGS exclusive SG/AC merch.)
On a positive note, I'll have 2 extra Cost free of the 18 cap for anything else I can fill in with.