I created my own star system for a custom race. You can create something new from scratch that is not selectable in the menu, but totally compatible with all mods, if you create the custom race XML first. (a tad bit more work, but the option is there) Anyways, the files you want to edit are: PlanetDefs and StarSystemDefs for the basic coding. A 26 tile planet could be troublesome, but if you are content with the Iconia artwork, that planet should work fine. (just copy all the art from that planet for your new homeworld) The planet surface artwork is tied into ContinentDefs.xml and has an impact of how many tiles is available on your planet, but Iconia is afaik, the biggest potential planet there is. (46'ish+ total with the Ultimate Terraformer is possible)
I'm also hoping to add at least 1 of each of the mining nodes to the system.
That could be the most troublesome part sadly. Since the generation is mostly random and with your specifications you would be running out of space in your system very quick. Also, I don't think you can make the planets you create yourself be resources, but I could be wrong, and there might be a workaround. Durantium on asteroids should be easy enough though. That is always plentiful 
I also seem to recall someone creating the resources on the planet surface itself (like the trade stuff you get there by default), you could look into that maybe?
Did you think about balance?
Seems to me he wants to just play with one huge solar system and simcity there. That is not as easy as it sounds if you play on a big map on harder difficulties.
