It is not against Tango guidelines to have an angled perspective http://tango.freedesktop.org/Tango_Icon_Theme_Guidelines#Tilted_Table I think that Tilted table fits perfectly for the 3DBox tool icon perspective.
I use this one http://img129.imageshack.us/my.php?image=draw3dboxwh8.png because the one that came with tango really looked confusing. The outlines render sharp in svg, my png export is blurred.
About the basic tools same color: rules and guidelines are there to protect the usability, but when it's evident the rules are going against it we shouldn't follow them. Maybe it's just a matter of habit and with time I (and others) would get used to same basic shape color, but now it seems having different colors helps me find them more quickly. It's much easier to distinguish between red and blue than between a circle and rectangle of the same color (and at that size). It does seem logical to make them same color because it's not the color that's the message of the tool, but then why is the bucket blue with yellow ink, or gradient icon green? If you make basic shapes all same color why not others as well (like Photoshop). Anyway I hope all this makes sense, I still vote for different colors.
If you decide to go with the Tango set, all of the toolbars must be Tango. It would probably be the worst option to mix icon styles.
I do agree that the main tools toolbar is the biggest issue, maybe there are ways to fix those issues. Here's a small part of the main toolbar as I use it, I'm one of those who think Pen, Pencil and Calligraphy look weird "bottoms up" so I made my own versions of 2 of them. In GIMP Pen is upwards, while Pencil and Paitbrush are downwards. Is Pen so different it needs a different orientation?
Mihaela aka prkos