Corrected entry: Assuming that in Series 2, Zordon gave permission for (before they were Rangers) Rocky, Aisha, and Adam to enter, Goldar and Rito shouldn't of been able to go into the Command Centre. Thoughout the series, it has been stressed that no-one enters the Command Centre without a Power Coin - this was even one of the subplots of Green with Evil, in Series 1.
Corrected entry: Billy uses a remote control to pilot the Falconzord (a Ninjazord) to assist the Aquitain Rangers. However, when Goldar destroyed the Rangers power coins in "Alien Rangers of Aquitar", we are told that the Ninjazords were now inoperable.
Correction: The Falconzord is special as it can combine with both the Ninja and Shogun Megazords. Billy was probably controlling the Falconzord as a Shogunzord, thus not needing the Falcon Power Coin.
Correction: The entire subplot for Rito and Goldar during this period involved them getting through the Command Center's defenses using an ancient map, thereby gaining access to the Command Center.