The AttackRides are nearly the same. While BurstRide replaces FinalAttackRide. KamenRide is the KamenRide: Ganba (Decade version) card and FinalKamenRide is the FinalKamenRide: Ultimate Ganba with Ultimate Ganba's face.
Yes, but Ganba is the 'Original Decade' before Decade even debut. The cards could have exist during the time when Decade was leader of Dai-Shocker, before Kamen Rider Decade started, which could have happen in 2007 of Decade's World at the same time as Den-O without being years ahead or behind.
First, you need to figure out the character to put in your lore. Next, Compare that character to your characters in terms of character traits and abilities. Last, try to write how your characters affect the story.
Short version (main riders): Den-O, Kiva, Double, OOO, Fourze, Wizard, Gaim, Drive, Ghost, Ex-Aid and Zi-O would get along okay, but might not like his ego. Build would be okay, but their egos would clash. I don't know about long, sorry.
He would transform into Ganba with the Decadriver, but if he transform with the Neo Decadriver, he still transform into Ganba, but would also have access to his powers in Ganba 2.0 and Ultimate Ganba without transforming into them.
Is just a monster or does it have a year and time? If its the first one then possibly summoning the monster, if its the second one he would be transported to the time and place, save him traveling by DenLiner or cracks.