Definitely Dib and Dipper. They are both curious and into the strange and supernatural. Dib with his alien - hunting skills and Dipper with his knowledge of the Gravity Falls mysteries would make a great pair.
In such a crossover fanfiction, Gir could get into all sorts of trouble in Gravity Falls. He might mistake a Wendigo for a new type of snack. Meanwhile, Mabel could try to befriend Zim, thinking he's just a really odd - looking new friend. The story could involve a race against time as Zim's evil plans start to affect the delicate balance of Gravity Falls, and the local characters have to team up with Dib to stop him.
Often, there's a theme of self - discovery. As Female Zim is a different take on the character, she might be on a journey to figure out who she is in this new form. Also, the theme of adaptation is common. Just like in the original where Zim has to adapt to Earth, Female Zim may have her own unique ways of adapting, whether it's to Earth's environment or to the reactions of the other characters around her. And there's also the theme of power. How does being female change Zim's perception and use of power? This is something that many fanfictions like to play with.
It depends on how you look at it. Some fans consider the comics canon because they continue and enrich the storylines from the show. However, others might have reservations based on personal interpretations.
You can start by finding a collection or anthology of Invader Zim comics. Many bookstores or online platforms sell them. Then, just open the book and start from the first page.
Well, it's a bit hard to give an exact number. Different publishers and creators have come up with various Invader Zim comic offerings over the years. You might have to do some research to get a precise count.