So... when I first installed 6tag last night, it sucked some battery, heated my phone, and pulled some data. The data doesn't surprise me, it's downloading a lot of pics and stuff. But I made some changes which seems to have helped the battery and heat.
First, go into 6tag settings and flip to the general page. At the bottom, there's a switch to activate the background agent. Turn it on, then turn it back off again. Until you do that, it won't show up on the phone's background tasks page. Once you've done that, go to your phone's settings and flip to the background tasks page. 6tag will show up; make sure it's blocked. Also tap it and uncheck "Turn background tasks back on for the app the next time I open it".
I then restarted my phone just because it runs Windows, but you probably don't have to do that. Since then, 6tag has neither drained my battery or caused heat issues.