I thought it was only the Meheri who speak south Semitic. I'll research later. Also many Gulf Arabs have Persian and Baloch ancestry. Genetically speaking, I don't think Arabia is genetically homogenous.They have some minor ethnic differences though. North-Central Saudis were the founders of the Arabic language (a Central Semitic language with its closest sibling being Hebrew), while South and Southwest/Gulf Saudis & Yemenis were South Semites (different linguistic category), historically.
Arabic replaced the previous languages of Yemen. People forget this.
So there is a minor ethnic difference between them. But overall they are similar.
This is a map of tribes of Arabia in the 7th century