Huh! Sorry, I missed that. I was looking at Terram guidelines, but under Herbam, as you say, there is an explicit L4 Guideline to turn plant matter into stone or metal, as long as you have a Terram requisite (this is what Rope to Bronze uses). Looking at the Animal guidelines, I think L5 "Change something made of animal products in a major unnatural way" is appropriate, so apparently animal matter->metal is a little harder than plant matter->stone. I think the Terram requisite would still apply.
But, if you're using those guidelines, your primary form is Animal or Herbam, and Terram is a requisite. I believe that means you would need two separate enchantments, one for each form. Whereas the approach I suggested above, though a little harder, can be done as one Terram enchantment with Animal and Herbam as requisites. So that might still be a better option.
In both cases, I would still charge an extra level for turning it into gold instead of bronze or other non-precious metal. But that's only on the basis that working with precious metals seems to be harder in other situations, I don't see an explicit rule about this particular case.
I know your new question is harder, because the Forums have a lot of debate about both Magical Focuses and Teleport. We play that a ReCo teleport spell will bring your clothes etc along. By that analogy, teleporting a chest should teleport its contents as well, even if they are Auram or Aquam. So I'd allow that to work. (But other groups explicitly say that a ReCo spell needs Animal/Herbam as casting requisites, or you arrive naked. Or naked, except for the clothes you made out of the skin of your dead enemies, you creepy weirdo.)
Personally, I would allow you to use your Focus to design a Terram spell that only works on items inside your focus. So you could do a spell to teleport anything made of metal (and only made of metal) and it will be a lot easier than a general ReTe spell. By analogy with the "clothes of the mage" rule we use, you could then teleport a sword, even if it has cloth on its hilt, or a metal box, even though it's been pained with plant dyes. Etc etc. But I wouldn't let you teleport a human being, even if you put them inside an iron maiden device (recommendation: remove the spikes before experimenting with this idea).
Similarly, your could devise other spells that will only work on metal items. I'm not sure what you mean by "without adding any size magnitude". If you mean "the item must be no bigger than an individual", I agree: nothing about a Focus lets you ignore the size-of-item rules. An individual of metal is up to a cubic foot, so your chest can't be very big unless you add levels for Target Size to it.
Take all this with a bit of caution! I'm doing my best to help, but I may still be missing things.