I have queried that rule in the past as I found it confusing considering full combat arenas should be able to hold all types of spinner, and most spinners would likely exceed both the 20% and the 500rpm points on a regular basis. If I remember rightly, the response I received was that these stipulations only applied to Grade 3 arenas and below (aka, arenas that are not designed to hold robots with spinning weapons), so if building for a full combat event you'll be fine; I think it's more to regulate something like 360 degree electric lifters. However as you are new builders, pics and specs sent to the EO won't go astray.