Hi @eigood, @jbus,
You can freely share your modified version to work with Yahoo Finance, my plugin is released under GNU GPL so you can modify it and redistribute.
The problem is that www.remarpro.com guides does not allow to plugins use Yahoo Finance data. If you wish, I can digg and screenshot my communication with WP.org plugin review staff regarding that.
So, plugin not hosted on www.remarpro.com can use Yahoo Finance ??
Btw, I sent question to AlphaVantage.co support regarding change, change percentage and last trade datetime data not provided by their API. If I get positive feedback with solution for those details, I’ll update plugin. And yes, there will be field in plugin settings for AV API Key. Terms of use for AV is open and does not restrict usage in plugin for www.remarpro.com repository.
Regarding IEX Trading, check legal requirements to use their API Terms of use.