The GPL requires that any derived works are also licensed under the GPL. For the purpose of WordPress plugins and themes, that means any elements which rely on WordPress (ie, use or extend WordPress classes and functions) must also be GPL. You are allowed to license other elements not reliant on WordPress (eg CSS, images) under different terms. Since you’re talking about including external code in your plugin, the external code (ie niftyPlayer) does not have to be GPL, as while your plugin may rely on niftyPlayer to work, niftyPlayer does not rely on WordPress to work.
It’s also worth noting that the MIT License is compatible with the GPL, so it should be possible to produce a GPL-licensed plugin using MIT-licensed components.
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