See, this seems a vastly more plausible approach than most, as the Goths were literally the other side of the world from Egypt.
1) The Crimean Goths aren't Goths? I'm sure they'd have been surprised since they were the ones who stayed put when the Visigoths and Ostrogoths went west.
2) That assumes we have the Arabs - or even some of the issues in the West. There are huge butterflies - from a possibility of not needing Justinians Restoration.
But assuming they do come to dominate the province (I find that unlikely, it seems more likely they'd be one of a few significant demographic groups in Egypt - Goths, Greeks, Egyptians, Arabs, Berbers?) but assuming they are the major group that supply manpower. If they dominate the trade routes (I would expect that more of the Greeks and Arabs, rather than Goths), I could see them setting themselves up in negotiated fortified trade posts - in fact, that sort of self-imposed isolation/seperation would likely characterise them.
What IS interesting however, is that if they maintain their Arian Faith (unlikely, but a cool option), is that the Arians control Alexandria!