Ok, great news!
First, another discovery. Adding a planet size smaller than tiny does cause the moons that are specified as tiny to come out as small as you guessed it might Puke.
But adding the new planet size has no effect on the player planets. An "average" start results in a medium planet, and a "Good" start results in a large planet. The same as the stock files.
I modified the systemtypes file and every where it called for Huge I changed it to Large, and every where it called for Tiny I changed it to the new smaller than tiny planet size. The results were better than I expected.
The planets that are specified as Large come out as Huge, like they are supposed to be anyway, and the planets(moons) specified as micro come out as tiny, like they are supposed to. So we end up with the new planet size and everything else exactly as it should be. And no need to add any new Huge planet sizes as you suggested Puke.
The game does add a few of the new planets in a random map, Typically one or two for a small map, and 4 to 5 for a large map. I don't think that is all too bad. Even if a player got lucky and had one in their home system it's not like it would be a tremendous advantage or anything. Actually it could be a slight disadvantage because it would take the place of a larger more usefull planet.
Geoschmo