It's generally not okay to take tiles from other fangames, and is considered stealing.
Let's say I do anyway, because I think that it's not stealing if I copy their credits as well; here's some real issues I might run into:
- Dev made the tiles themself, and because they're involved in every part of developing the game, didn't bother to list their name under every credits section → I've failed to credit correctly, so it's just stealing
- The tiles were a private commission made from scratch, not a rip or remake from Pokemon games → There is no situation where I had permission to use these
- The developer's credits were wrong, or they stole the tiles first → I'm a thief no matter how you look at it
- The tiles were published under a specific Creative Commons license or have other rules about how they can be used/edited/distributed → Even if I wasn't considered a thief before, my ignorance has made me one
You may not care about having a reputation in the community of other developers (though believe me, being known as a thief is a hard one to shake off) but it is pretty cold-hearted to disrespect our peers in a niche community like this; it's only because of the kindness and willingness to share with each other that we have so many resources to use. The consequences of stealing may not be immediate and may not feel personal, but still harms the very community that built our game.
It would probably be best to contact the Insurgence devs directly and get their permission - I'm not sure that they're on this forum.
For what it's worth, in my time as an RC admin I leaned it's a common mistake for newbies to take from [almost allways Insurgence specifically but sometimes another big game like Reborn] and simply not realize that's considered stealing, so I don't want OP to feel too bad about making a mistake and having to correct it.
This happened regularly enough though that there were multiple people who claimed they
did have special permission from the Insurgence devs, and over the years I'd allways call their bluff by contacting a relevant team member and surprise- no one had ever gotten special permission.