I made a template for the Guardian and Elder Guardian if anyone's interested. Their texture layout is pretty confusing...
instead of using a normal layout for the body cubes, Jeb kinda just splattered them all over the place. One of the inner cube sides actually cuts straight into the tailfin texture, forcing you to either draw both the tailfin top and bottom the same color as the inside of the cube, or (in the case of the default texture), leave a really awkward looking cut in the inside of the cube. Luckily, none of that is visible if you keep the entire texture opaque.
Download's on my website, here. It's a .zip file that contains a png, xcf, and psd of the texture, with all the cubes on individual layers to make editing and previewing super-easy. All the other information you need is on the website page.
I haven't messed with it yet. Is there a pattern to colors you chose for the faces?
Green is front, White is back, Yellow is top, Pink is bottom, Turquoise is the guardian's left, and blue is the guardians's right.
Accurate for all the cubes except for the side cubes, which are not mirrored, so basically the guardian's left and right side cubes use the right side texture.
This guide has been included in the "The All-Inclusive UPDATED Guide to Texturing" page. Thanks Ringoster for making this.
There is a link on that page that links to this page. Thanks for creating a guide to the Guardian's mob texture layout.
Here is the link to where in the texturing guide it's been added: Click Here (It's under the "Entity" spoiler, at the bottom of that spoiler)