It really doesn't matter what clan the imam came from as long as we know he is of somali origin. The imam didn't fight along, the futuh al-habasha was a team work with an army made out many different clans named in the manuscript. If one of these clans didn't participate the imam might not have scoured such an overwhelming victory against the solomonic dynasty.
Now if we wanna know what clan the imam came from for the sake of knowing i might have some evident based answer for you. The futuh as valuable as it may be it didn't talk much about the imam and his origin, it didn't even state the full name of the imam but we have another manuscript from the same time period with useful information in regard to the imams origin. You see in the 16th century 500 years ago there was a man called ba alawi and he wrote a number of manuscripts. One of these manuscripts carried the full name of the imam and went like this.
As you see the name of the imam is ahmed ibn ibrahim ibn omar ibn ahmed.
Now this is a vary important account of the imam's name but it's doesn't go further than this and we still haven't answered the question of where the imam came from. For our next clue we need to go to the 20th century to a man named ahmad a-shami a harari man interested in history and literature. He apparently had a manuscript kept by his family for generations. This manuscript also had a genealogy of the imam and it went like this.
Here is a translated version of the same text.
As you see both texts agrees with each other and the manuscript from ahmad al-shamsi expand on it were it mentions ahmad who they call a somali prince. The accounts gets a bit jumbled up and repeats gran a number of times because the original manuscript didn't show the name properly so we don't know what is writing but the final name is clear an it say karanle.
This means the imam is from the karanle clan. Notice how ahmad al-shamsi source is different from that of ba alawi and it's only concerned with the imam lineage. Aside from being two different sources that agrees with each one manuscript is 500 years ago and the other was kept by one of the families of harar. This confirms the imams lineage and genealogy as being a somali from the karanle tribe.