In fact, it's Played for Laughs and lampshaded when Daigoro Oume and Shiro Akebono bump into the Gokaigers at the same time: the Gokaiger mistake them for Gavan, but Gai insists that there are actually a lot of differences between the two while he's looking at both of them.
Earlier in Ghost Ship, it seems modifications were done to the Gokai-Oh suit to make the evil Gokai-Oh suit. As a result, the two Gokai-Ohs are only seen together in CG, unless only parts of one interacts with the other, and never the same piece (right arm with right arm, etc.).
Gai's actor, Junya Ikeda, stated in his blog that when he was a child he wanted to be the Kiba Ranger. In episode 19, the first thing Gai does when he is allowed to summon Ranger Keys is to summon the Kiba Ranger key.
In episode 29, when Ahim dons a schoolgirl disguise, she says she's from Saitama and needs directions. Her actress, Yui Koike, is actually from Saitama, herself.
Joji Nakata, who played the Monster of the Week Zaien in the Liveman tribute episode (#30), was the same actor who played the lead antagonist in that series (Professor Bias). Both were also responsible for turning a former friend to the enemy - Kenji, Rui, and Goh for the Liveman and Sid for Joe, though they were done in different ways.
The reason that you never see more than three Sgormin on-screen at the same time is that Toei literally only has three Sgormin suits.
This may also be the case with the Dogormin (read: red Sgormin) since only two of them appear each time. Thanks to a Whip Pan between split-screens, we see four Dogormin on screen in #49, two over each shoulder when they come to offer backup to Insarn.
May also be the reason Ahim was Green Flash. Toei may've known that they were gonna get Dai back but no pink rangers and made the suit for the All-Green change in #43, killing two birds with one stone (It's assumed most of the gender changed suits just add a piece of fabric for a skirt for male to female. However, some of the teams couldn't be done like that, like Flashman, so Toei probably had to be judicious about when to gender change those suits.)
If you pay attention, you'll notice that there were a few gender-switched costumes that showed up over and over. Green Flash is one, but female version of Magi Red and Ginga Red showed up too.
Reportedly, two of the head writers nearly came to blows over Gai Yuki of Choujin Sentai Jetman in the Great Legend War - one of them, an original writer for Jetman, didn't want him to return, but the other was insistent he did.
There were also rumors that GokaiGreen was going to have to be recast or otherwise replaced after his actor Kazuki Shimizu was injured when they were filming near where the early 2011 earthquake struck (thankfully, this was just a rumor.)
To be fair, the earthquake did just fine messing up production on it's own, causing 199 Heroes to be postponed from 5/21 to 6/11. The series was only postponed a week (earthquake occurred before the Dekaranger tribute). That not getting into the fact that the earthquake would be on everyone's minds and they likely had their own worries about it.
Dead characters from previous seasons were apparently brought back to life to fight in the legend war. Who brought them back isn't clear. Apparently Rio and Mele being killed in the series and then revived to fight Long in Go-on vs Geki is mentioned in the pamphlet as something that happened, so this adds a bit more evidence that Gokaiger incorporates everything as "canon."
There was a massive battle that happened prior to what we saw in the Legend War where over 100 robots/giants fought against the Zangyack fleet. Many of the robots were destroyed or killed during this battle. This was also mentioned in an interview with the Director, Takemoto and was posted on 2ch long before the pamphlet came out. The only hint towards this in the actual movie is a brief shot of a destroyed Gosei Great during the opening battle.
The lead writer's initial plans for Don's backstory would have been that he was an orphan whose scientist father was killed by the Zangyack. He ultimately left the character backstory-less once he realized he preferred characterizing him as a Ridiculously Average Guy. The only remnant of this scrapped story that did make it in a sort of way is that Don's homeworld did get blown up by the Zangyack. It gets only a passing mention when he talks about being able to restore his homeworld with the Greatest Treasure of the Universe in #49. And, aside from the whole thing he fabricated about being an amnesiac dragon-slayer, not much is said of his backstory except for the details on how he joined the crew.
Early info for Gokaiger vs. Gavan said that Warz Gil would appear in the Makuu Castle prison scene, but this turned out to be false, presumably because it would be a Plot Hole for Zangyack's own prince to be there.
In episode 5, when the four Gokaigers become Go-ongers, Gokai Yellow used the wheel designed armbands of the Go-onger suit as a weapon, which is exclusively used in Power Rangers RPM.
The entire scene when Elder is discovered in #8 pretty much homages a similar scene in "Handshake", even substituting billiards for baseball.
Most of the time, when the Magiranger powers are used, Ahim will use Vida's 'big pink tornado of windy doom' technique rather than Houka's 'turn into something small and cutesy' technique, though it could be Ahim simply having a different personality from Houka.