You can use and modify shaders from others however you like for your own renders - however you may not redistribute them for free or commercial items without the express permission of the original artist.
Pretty much, if you’re not selling or giving away your modified textures you’re fine. You CAN sell renders or animations.
Many freebies come with a “non-commercial use” or “contact me first” statement. This is usually because someone else own the rights to the character, etc. This wouldn’t prevent you from doing what you asked about, but you would have to keep track of the modified product an non-commercial etc.
To summarize: You can probably do this and avoid any trouble as long as you follow the rules for the original stuff.
Note that shadres made with Shader Mixer will be included in a preset, so you can’t usually share those. The Visual style shaders use a custom script to create presets that are shareable, but you can’t use the File>Save as versions.
Actually back when I think it was poseworks the creators of the pw shaders site was fucntional people would upload modified or custom shaders built using their shaders. i remember one of the being a really good hair and fur shader. But you would need to have their shaders installed for them to work. These were how you say shader presets
Hmmm. Well, there will be a bunch of brick definitions in the preset if you open it in a text editor, or if you take the shader into Shader Mixer you will get a non-standard network. Also the label at the top of the Surfaces pane won’t be one of the standard shaders (DAZ, pw or om). But I’m not sure there’s a guaranteed indentifier.