Look in the QuadrantTypes.txt in the data folder. There you can find the percentage chances of the different system types. Current setting for ancient is 15% black holes, and 8% for each of the 5 nebulae (storm1-5) types. If you change the numbers make sure they add up to 100 again.
I cannot find the nebulae mod either, but all you need to do is some copy and paste in the SystemTypes.txt. There currently are 5 storms. You need to make 25 instead (1a ,1b, 1c, 1d, 1e, 2a, 2b, ... or whatever). Make 1a-e the same as the old storm1 in all parameters except for the bitmap (where you put the 5 different bitmap types). A simple Version of this would be leaving it only with 5 storm types and giving them all the same bitmap (all storms would look the same).
If you go for the 25 storms variant, you need to change the QuadrantTypes again of course and give storm 1a,1b... the numbers that you want.
Was that confusing enough?
Hope that helps.
Rollo