There is already a thread on here that has a graphic displaying the limitations of each mash tun size. I'm on my phone right now, so I can't quite paste the link. Pretty sure the limit on O.G. Is around 1.06. I'll post the link when I get to my computer.
Also, using a bottling bucket as a HLT would most likely be bad idea. Water of that temp will not stay at their temperature due to a lack of insulation. In addition, I don't think the plastics of the bottling bucket can withstand those high of temperatures