There's 4 T20s in the series against South Africa.
We have about 5 opening options in the team now if Fakhar is added:
Fakhar Zaman, Sharjeel Khan, Babar Azam, Mohammad Rizwan, Haider Ali
Right now, we have Babar + Rizwan opening the innings, which is not bad given that they have the credentials and performed well in the PSL as well. However, Rizwan is kind of like Wriddhiman Saha in SRH, he either plays as an opener and does well, otherwise he's pretty useless anywhere else in the order. Babar on the other hand can be dropped down to 3.
Hence, that gives us an opening partnership of Fakhar/Sharjeel/Haider + Rizwan. Personally, I'd play Haider in the middle order seeing as he didn't handle opening the innings well, and he played a few decent knocks down the order as well.
Therefore, the remaining 3 players for the opening positions are Fakhar, Sharjeel, and Rizwan. Anywhere else in the order, Rizwan becomes useless. So if we accomodate both Fakhar + Sharjeel, it would be at the expense of Mohammad Rizwan in the T20 team.
Fakhar is a dynamic opener, but he's inconsistent. His strong points are his ability to hit sixes, and the fact that he can bash the daylight out of spinners. He is also a good runner between the wickets.
Rizwan's strengths are his current great form, and also the fact that he provides stability at the top of the order. He is also a wicket keeper, allowing us to accommodate more bowling options in the team.
Sharjeel is extremely explosive, and he is in good form right now as well. He can clobber pacers and spinners alike, but his fitness holds him back from rotating strike against accurate powerplay bowling.
If we consider the middle order for a second, we have:
Babar at 3, Hafeez at 4, Haider at 5.
Sarfaraz could be added to this list quite easily, meaning that Rizwan wouldn't be necessary at the top if we are willing to accomodate Sharjeel and Fakhar.
Personally, I'd play Sharjeel + Rizwan for the first two T20s, and if it doesn't work, I'd play Fakhar + Sharjeel and bring Sarfaraz in to keep wickets.