The Silent Imperium's relations to non-Dominion factions will need to eventually be worked out since I don't my faction's whole existence to be 'those things that really hate the dominion' despite that oath they took over a thousand years ago. A thousand years is a lot of time, they are bound to have gotten wrapped up in other affairs by than, especially with their tendency to fumble about.
I could see the Terminus trying to help some of the Tikalli when they made contact. Granted the Terminus likely doesn't have a particularly pleasant view of the Silent Imperium itself, seeing how the Simulacrum put a major emphasis on personal freedom and liberty. They'd probably get along far better with the non-Pact aligned Tikalli; hell, I can even imagine the Terminus having given some of them a garden world. After all, worlds like that are more difficult for the Simulacrum to live on.