You have changed the requirement for a Silicon Race to build a City from Durantium to Promethion, however, it still says Durantium when you look at Race Type Silicon Based for any Silicon based Faction.
If you play any Silicon based Faction and you create a Colony Ship, you can colonize a Frozen world without any ability or research. Basically, the Slyne can colonize the frozen planet in their system game turn one if they had a Colony Ship. I was not sure if this was intended, but, if it is then it should be part of the Silicon description. If this is the case would the only value for a Silicon Faction to take the Adaptable trait would be to immediately get Aquatic and Barren worlds? Personally, I could see Silicon based Factions being able to colonize Barren worlds, but not really Aquatic or Frozen. This ability gives the starting worlds for the Slyne a huge advantage since they basically get a size 11 and a size 9 worlds very quickly which total 20, when the total should add up to 16 like other home worlds combos.
Lastly, it seems to me that even though Silicon bases cannot build a farm, why should they be able to take advantage of fertile lands? What I mean by take advantage is that they can use the "food" they have to build a Colonization Center, which seems really odd. I say, take away this building from their options and give them the ability to colonize Barren worlds. Also, the Onyx should probably start with at least one Promethion around them instead of Durantium all the time.