The OP says not to raise the number of themes above 40% of the total number of constellations. Is there any particularly important reason for this?
I'd like to try a sector with 100% high danger remnants and I'd like to know if this will break anything. I will also reduce all other themes to 0.
You can do that yes, but the problem is that no ruin/derelict theme means no stations/caches/mothership to explore/fight and loot.
As for the 40%, it's more like "1 theme = more than 40% of you total constellation count". If you do it, the remaining themes will likely not have enough constellations to populate, resulting in less constellation per theme than specified in your settings.
So for example: constellation count of 150, 15 derelict motherships, 120 constellations for ruins, 50 for remnants. Since mothership takes 5 constellations each, you'll get 75 constellation for derelicts, 75 constellations for ruins, and none for remnants.