Im playing Admech/imperial knights (seperate detachments) and I'm wondering, if my warlord is a knight, am I able to use admech strategems?/is the trait in effect?
You can find all the information you need regarding the stratagems in your codex - right on the page with the stratagems. I'll quote from the Codex "Astra Militarum", p. 134 (of the german version):
Wenn du eine Armee in Schlachtordnung hast, die beliebige Astra-Militarum-Kontingente enthält (außer verstärkende Unterstützungskontingente), hast du Zugang zu den unten beschriebenen Gefechtsoptionen; du kannst also Befehlspunkte ausgeben, um sie zu aktivieren [...]
Very Loosely translated:
If you have an army that is battleforged and contains any Astra Militarum detachments (except whatever the english name for the detachment that allows you to just take any single unit you want for the price of one CP is), you gain access to the stratagems below. Meaning you can use Command Points to activate them
This text also appears in other codexes (I checked my codex Chaos Space Marines) and in the Psychic Awakening books (I checked Glaube und Zorn, which is the german version of Faith and Fury)
This means that as long as you have at least one full detachment from a specific army/keyword, you can use the stratagems. Note that most (if not all) stratagems are pretty much limited to be used only on the faction they come from (or even more specific units) - Your choice of Warlord is completely irrelevant for this, you have access to both AdMech and Knight Stratagems no matter which Warlord you choose.