Been reading over Fiendish Codex 2.
Basically, Silas has had dealings with a Harvester Devil.(FC2 pg117). Through many months (or years) or speaking with Matukal, Silas eventual signed a Pact Certain (FC2 pg23) and declares allegiance to Mammon. Silas is after personal power and the wealth that comes with it.
As far as game-balance in regards to other starting characters, there is nothing in the Codex which states the gained benefit from the pact is immediate. In the case of money (depending on the level of corruption that Silas must attain), who is to say how long a devil is to distribute a reward, as long as he does so according to the pact.
To the GM(s), I think it would be best for you to devise the diabolic terms and conditions of the Pact. Silas will have no problem gaining corruption points (FC2 pg30) due to his evil nature.
As for his worship of Zilchus: I don't think Silas is part of the Mammon cult yet. Zilchus and Mammon both share the Trickery domain, so I may choose that for one of my domains. And after signing the pact, Silas is immediately Lawful Evil, which wouldn't matter much as a cleric of Zilchus.
Other possibilites: To attain his position as leader of the church of Zilchus, the former high-priest would have to be removed. To facilitate this, Silas would hire a 'hit man' to keep his hands as clean as possible. It is quite possible that this hired killer would be from the Fellowship, thus forming a relationship with the group.
Silas' reputation for extracting information (whether through confession or torture) has attracted the attention of a Fellowship member who needs a favor of their own.
Those are some of the ideas I am having so far.
On another note, I am having a devil of a time finding a suitable portrait. I've been using Google image search, and am having no luck. Any ideas?