Before I directly answer your questions, please let me state that I intend to make a similar SupCom mod for both Civ 4 and Civ 5.
Now, to answer your specific questions:
1. Yes, I have modding and programming experience.
With regards to what is required to mod Civ 5, I have experience with Lua. With regards to Civ 4, I am only tangentially familiar with Python, but enough to pick up programming in the language in short time.
I am also a member of an extensive mod team for Supreme Commander 2:
http://www.moddb.com/mods/revamp-mod/
With regards to general programming experience, I am proficient in C++, C#, and WinForms. In fact, one of my projects within the above mod team is to create a Mod Creation Toolkit to make it easier for anyone wanting to mod Supreme Commander 2. The entire toolkit is GUI-oriented and much of the code is automatically generated for the modder. I have working prototypes and I intend to publicly release my toolkit later this summer.
For example, have a look at the following screenshots:
The main section in the middle with all the squares are locations for items in a research tree. The modder uses the tabs on the left-hand side to locate any of the game's units, structures, and upgrades and then drag-and-drop them onto the grid. The text boxes on the right are made available for the modder to manually change values in the corresponding Lua files.
1.b) No, I do not have any experience modding Civ4. However, I am willing to learn the necessary in order to mod it.
2. SupCom and SupCom 2 models are all available. Perhaps just some tweaking in order to get them to work correctly in the Civ games, but they are there. You can see at the above website that modelling will not be a problem. As for Total Annihilation, I would like to do that mod for Civ, but not at this point. I am creating my own TA models for use in a TA mod for SupCom 2, so that material would eventually be recycled.
3. The major concepts are worked out, but I do not intend on relying on any existing mods.