The helmets used for the A-Squad Rangers (the ones admired by the starring Power Rangers around Jack and co.) are the helmets used in Power Rangers in Space (1998). Only the black PRiS-helmet was repainted to green.
According to Jason David Frank who played Tommy Oliver from previous Power Rangers series, he was never contacted to return for the annual team up of Rangers from this and the previous series (Power Rangers DinoThunder (2004).
Bullets and missiles are no longer allowed to be shown in Power Rangers under new guidelines from Disney/ABC. In the original Japanese footage the Red Rangers guns and the Blue Rangers Zord both fire bullets at the villains. However in the US version these are edited to look like lasers instead. If you look closely you can sometimes see smoke coming out of the Red Rangers Delta Blasters, even though he's only supposed to be firing lasers.
Disney wanted to save money and didn't want to hire a seventh starring actor, so it was written in that, Sam the Omega Ranger, as a side effect of time travelling, he was either stuck in his suit or transformed into a ball of light. This meant he was played on set by a stuntman in a bulky helmet and had his dialogue recorded later in ADR, making interaction between him and the other characters incredibly awkward. Apparently, the crew was so frustrated by this that they tended to write around Sam as often as possible; they probably would have written him out entirely if not for stock footage constraints. Bruce Kalish has said that Sam could have been handled better.
The trivia item below may give away important plot points.
This is the first Power Rangers series to feature a team of fully evil Power Rangers. While previous series had featured evil Rangers they were usually clones, under mind control or otherwise mirror images.