Re: http://premium.wpmudev.org/ Legitamate?
I think your last question is moot. Developers will very often create plugins and offer them free under GPL license. Just take a look at the WP plugin repo where you’ll find over 13,000 free plugins: https://wordpress.org/extend/plugins/
But anytime you request something be created or custom-built, it’s best to offer some form of compensation; whether you want some coding done, or a deck built in your backyard