Is Kingdom Hearts in Anyway linked to Final Fantasy?

Well Kingdom hearts is produced by Square Enix who are the guys behind the Final Fantasy Series. And most of the things you will find there are not really easter-eggs but rather the point of the game.

It is officially a crossover of Disney, Final Fantasy and The World Ends with You. Of course it still has an own story and characters but often you will find characters from different universes.

It's believed that the Final Fantasy characters in Kingdom Hearts are from a parallel universe. However, Kingdom Hearts is a separate franchise and the FF characters are really just cameos. The Final Fantasy characters actually don't impact the story for Kingdom Hearts that much. Kingdom Hearts focuses more on Sora and his friends than Cloud and all the other FF characters that appear in it. They did put in small similarities to some of those characters. Here are some examples of what I'm talking about:

1. We know that in Final Fantasy VII, Aeris (or Aerith) is killed by Sephiroth. In KH2, Aeris is alive but apparently doesn't know Cloud. However, Aeris still supports Cloud.
2. Cloud changes his outfit at some point during KH2 and it's the outfit he wears in Advent Children (which is called "Cloudy Wolf"). Advent Children occurred two years after the events of Final Fantasy VII (or two years after Aeris died). It also mentions in KH that Sephiroth is the "darkness in Cloud's heart" but Cloud and Sephiroth are not related in anyway. But Cloud does become a Sephiroth clone between VII and Advent Children.
3. Zack Fair only appears in Birth by Sleep and is a younger version of him. After Birth by Sleep, Zack pretty much disappears. But in Crisis Core, Zack is older and was killed by Shinra soldiers. Zack's memories were also transferred to Cloud. This is why Aeris is a love interest for Cloud, because Zack had feelings for Aeris.
4. Squall is alive and older in KH2. But in Final Fantasy VIII, it is highly believed that Squall dies at the end of disc one and was 17 during the events of VIII. Squall is also older than Cloud and Seifer in KH.
5. KH2 hints that Squall and Cloud are friends or rivals of some sort. However the only Final Fantasy game where Cloud and Squall are in the same world is Dissidia Final Fantasy which is an arcade-style fighter game.
6. Seifer is a teenager during the events of KH2 and has a completely different fashion than what is shown in VIII. Seifer is also missing his gunblade and is friends with Vivi. But Vivi doesn't make an appearance in the FF franchie until Final Fantasy IX.
7. Vivi is also deceased and KH2 depicts him to be very clueless.
8. In the first Kingdom Hearts, Wakka, Tidus, and Selphie live on the Destiny Islands with Sora, Riku, and Kairi. But Wakka is 23 in Final Fantasy X (25 in X-2) and Tidus is only 17. Selphie only appears in Final Fantasy VIII. Tidus was also the battle tutorial in KH.
   9. Yuna is depicted as a "fairy" in KH (don't understand that at all) instead of a Summoner.

And those are pretty much the major ones. I know Rikku (not be confused with Sora's friend of the same name but different spelling) also appears in KH but I don't exactly know her purpose and Auron also appears in KH2 and disappears the same way he does in FFX. So the true answer to your question is: No, Kingdom Hearts actually isn't linked to Final Fantasy. And to further that a little more, there are no major references to Kingdom Hearts in Final Fantasy (with the exception of XV which makes a small reference to KH3 and the fact the 1.5 cover art was literally from a scrapped concept of Noctis from when XV was known as Versus XIII). Finally, even though Square Enix made Kingdom Hearts the rights to the game actually belong to Disney and the reason why is obvious. The Disney characters.