Covers and fonts for the Kindle Fire

[Shonky Code]

I've spent the last couple of weeks wrapping my head around coding a mobi via Notepad++ and KindleGen, and despite some likely horrible code, I have a result which looks great in Kindle Previewer and on my Kindle 4.

That is, except for the Kindle Previewer set to Kindle Fire. The cover (800x600px) only occupies about two thirds of the preview screen, if that, and the body and header font are the unattractive Times New Roman.

From snooping around here, I'm under the impression that I shouldn't set fonts, rather let the reader decide for themselves. Is that correct?

As for the cover, is this just KP being buggy? The standard Kindle cover size is still 800x600, >127kb, right?