First things first... I actually like TSLA buying SCTY and I'll be voting for this with my shares. I'll probably have to explain this in further detail, but the short version is that Tesla Energy has immense potential and with SCTY they have a great management team, sales and installation to scale Tesla Energy in a very fast manner. The few billion dollars they pay for SCTY now isn't a big deal considering the tens of billions of dollars in revenue they'll have from Tesla Energy in several years.
"Disinterested stockholders" are basically all stockholders minus insiders (ie., directors, officers, etc.). So, Elon Musk and the board won't be voting. And there needs to be a majority of stockholders approve this on both the SCTY side and the TSLA side. I can see the SCTY stockholders approve it fairly easily. But it's probably going to be a bigger challenge for TSLA stockholders to approve it (as evidenced by the general attitude against this on this forum).