I'm trying to delay getting assaulted by them as long as possible as my colony is in a bad spot.
Thanks!
AFAIK, only wealth and your storyteller settings. Randy and Cassandra both WILL eventually pop every event in the game, Cassandra more reliably so, even if you aren't particularly wealthy.
Mechanoids cost a certain amount of "raid points" per, so when the game can afford to send them, it will start sending them.
So while they're not "activated" by your wealth, they won't show up until you are at a certain wealth level.
I have had raids consisting of 1 scyther a few times, very early in the game. I think it can only happen on Randy. It is not common. This was on A17 or A18, don't know if it changed since.