It's not impossible that we could come up with a way of crashing less than 15 moving objects together to get an alternate RCT pattern.
Gliders are generally considered to be the "lowest common denominator", though, so adding complexity by allowing more types of spaceships isn't usually seen as an improvement.
... It also becomes possible to cheat: I suspect we could put together something like an "RCT8" if we allowed Corderships as well as gliders in the list of allowed moving objects that we start with. (2-engine Corderships' "engines" are switch engines, and we have to build four switch engines to get the RCT reaction started. Could probably just shoot down the extra switch engine with one glider, and go from there.)
