Indeed that can be a good solution for the bebers. However I think that you could have another two options:
-Create a "Independent Peoples Faction", like the one of Roma Surrectum II, inside which you can add many minor peoples or factions (Berbers, Tankhids, Vandals, etc.) with their own unic units (for example the Numidian cavalry in the areas nder berber control) in each of their respective areas. Besides with this first option you wouldn't need to appeal to the "amalgame factions", and so on you could include more specific factions.
-Amalgamate the Berbers, Nubians and Blemmyes factions in one "African Peoples Faction", and so avoid the odd into have in one only factions Berbers and Arabs (having in mind that except maybe in Egypt both peoples doesn't have important contacts until the Arab conquest of the VII and VIII centuries.
And below some more pictures of Late Phartians of Aventine which could serve you as inspiration: