I know tournaments require everything to be wysiwyg but this is also a hobby that revolves around models and conversions as well.. so where is the line? A lot of people like to use different Space Marine chapter specific models to represent other marine forces and that is generally considered to be ok. Does that only apply to space marines though?

Can I use daemonettes or dark eldar wyches to represent Death Cult assassins in a Cortez GK list that "counts as" Renegade Guard?

Can I use a Chaos Sorcerer Terminator Lord model to represent a Herald of Khorne w/bloodcrusher in a Daemon list that "counts as" a lone Sorcerer summoning the daemons?

I'm obviously not talking about using an Ork to represent a Fire warrior, but was wondering how people decide what the difference is between following the rules vs being creative.