Using resources from other IWADs

Hello there, I am planning on making a mod, but I wanna use some stuff from Heretic, Hexen, etc, Is using stuff from other IWADs not allowed, and if not, is there a way that can implement the resources if someone has the IWADs?

Using resources from other IWADs isn't normally a problem, as long as you're not including so much of the IWAD that you're practically giving away the entire game. Lots of people borrow resources from games like Heretic or Hexen. Lots of monsters on the Beastiary include or modify IWAD assets. (In fact, you'll even find monsters ported from Duke Nukem, among other games!) You just want to make sure that you're using those resources in a way that matches the Doom style, or at least in a style that's consistent within your mod.

I wouldn't recommend having your mod use IWADs as required resources. Not only would this make the mod unplayable to people who don't own all the required games, but loading multiple IWADs at a time is going to cause some conflicts. ZDoom is actually smart enough to handle some of them (like actors sharing the same sprite name), but you're still going to have some IWAD conflicts get through, not to mention possible conflicts with whatever you put into your mod.

Only really planning on using most of the items, weapons, decorations and monsters, is that fine usually?

It's pretty much entirely up to you and what you think is appropriate for your mod. There's really no technical limitations on what you can do with importing actors. You just want to be careful not to go overboard with how you use them. A lot of people frown upon the practice of randomly inserting Raven actors into a Doom environment, without taking care to use them in a sensible, thematically appropriate fashion--it tends to come off as a bit amateurish. But you can definitely get away with mixing styles as long as you're careful and consistent.