I need to cut pieces that will be study and not lose their shape.  I tried to use Modeling Chocolate, no go.

I purchased some gumpaste and was going to try for a mixture as I read that it works well

Then I found this.  It doesn’t look promising with the mixture. 

https://www.mybluprint.com/article/mixing-fondant-modeling-chocolate

Any thoughts on what’s the best medium(s) to use?

Thanks!

What are you trying to make, KatyN?  It will be easier to offer a constructive answer if I understand your goal.

Oh, sure!

  I’m constructing a bridge.  So I’ll need it for everything, the legs will be secured with larger pieces of the same medium and royal icing.

If I were making a bridge, I would use pastillage, which is much stronger than modeling chocolate, fondant, or gum paste.  It's strength and durability are its advantages, it drys much faster than other mediums is it's disadvantage.  If you don't want to use pastillage, I would suggest gum paste and give it ample time to thoroughly dry.  Personally, I would not build a bridge without some sort of internal support, probably heavy gauge wire.

Here is the recipe and how to make pastillage:

bump

Sedar Yener has launched a commercial line of his pastillage.  I can't wait until it reaches the USA.  Pastillage is the only thing that makes me nervous, although I have made and used it.