I'm not sure I'm necessarily wrong about the Planetary Defender fighter, though. Wookieepedia does identify it as being from the "Rebellion Era," which is as you described it. But that's only because the Planetary Defender fighter only appears in a game that's set in that era. In that game, a jerkwater planet called Tinoon, in the middle of nowhere, is seen using them "extensively." That means they must be in pretty wide circulation by the Rebellion Era, and almost certainly a decade or more before it. I would assume, thus, that's around in this time period, too.

As for the Golan III platform, I believe they're said to be around during the Clone Wars.

And as for the Cambrist shields, by "several" I basically mean enough so that if Brentaal is ever attacked by pirates in the RP I can say that these shields are present there. The actual number isn't all that important to me.

As for the Cambrist Shields, I do understand your concern, but rest assured these really aren't practical for planetary defense at all (capital ships can tear through them in seconds, for instance). The sole purpose of these shields is to have some sort of defense against Volus-style acts of piracy on civilian ships, freighters, transports, etc. over Brentaal. Basically, it's an attempt at a bit of balance in the RP because we've seen how easy it is for an RPer with a couple of pirate ships to show up at a planet and shoot up an obscene number of defenseless NPC ships before the planet has enough time to stop them. So, as for how many of these shields would be at Brentaal, it's hard to say; realistically, considering the volume of traffic that passes through the planet, there would be a hundred or more of these. But I don't intend to ever use more than 4 to 6 of them at any one time. Would that assurance be acceptable, in lieu of a total number?