If you create your own assets you can definitely use them, but you will have to provide them with your project as well.
Using external applications to generate models or images is perfectly fine, even if they're commercial tools. You would have to export from Maya to some file format you can import into your game engine of choice anyway and provide those asset files. So if you had your model as an FBX someone who doesn't own Maya may still import it into Blender if they wanted to...
As for subscription services... pay attention to their licensing. The assets you get from there most likely do NOT allow redistribution, so they're out!
If you used for example textures.com and paid for higher resolution versions and included them in your finished build, you could still list a link to which textures you used and someone who did not pay might get the lower resolution version of it instead. the game might not look as good, but would still be fully functional (more or less, depending on what the texture images were used for)...