[@Oraculum]
You have lots of creative freedom with custodians.  The major drawbacks is that I you have to take orders from PM or at least pretend to (though if you don't I will make IC consequences happen for your disobedience).

In terms of designs you can be biosynthetic, digital, logic breaking and surreal as you want them to be.  They can be everything from a monitor, to automatons enforcing long dead laws on every living thing, the beings that keep the machinery of Solomon, Ezra and so on functioning and so on.  

Basically the custodians reveal the ancients to be much less monolithic than first impressions of the sprawling ruins or mega engineering projects would make you think.

As for familiarity, they'd know more than the humans but still be in the dark on things even if they have a much better understanding of the nature of Solomon and the whole star system for that matter.  Changes are you have visited every world at some point if you are a custodian that can travel between worlds and have likely been around for some ridiculous amount of time (though custodians can also be generational in nature).