Is it ok to use 2D particles in a 3D world?

That's pretty much been the standard use of them for VFX like flame or weapon blasts all along. If it DIDN'T work in Godot that would be a serious feature deficiency.

It has 3d paricles, plus 2d particles. Looking at the tutorial in the manual, it don't work with 3d world. And i ran into another problem. The 3d particles tool set is so limited, that I would have to build all the tools in it for doing things like rotations and movement. So what I did was created a gdscript 3D particle script. I plan on updating it to use the MultiMesh Instance. If things seem slow. My gdscript was based on what I learned from learning about particles and shaders. What's different about it, is performance and the Godot GDScript tools. Such as rotation. Not only that, but I can use scenes, rather than .obj

could always render the 2D particles into a viewport/texture and display that on a mesinstance plain or such.

Oh, I misunderstood, you were asking about the 2d particle system, rather than 2d sprites as particles. D'oh. Keeping the sleepy avatar image the board gave me as a default was not wholly a matter of laziness.

I looked at the manual on viewport rendering. Good to know Godot can do this. I'm hoping I can use it for text display. Right now I have my own hand drawn sprites, that get converted into 3d sprites. The font doens't look that good but it works. But, if I can get the viewport working.