On Karachi, it is a Muhajir city, they built it, and make up the majority, but other communities are welcome, if they contribute. I generally find that Punjabis and Seraikis are good immigrants, they work hard, learn the local language and intergrate. Pashtuns, unfortunately form ethnic ghetoes, and refuse integrate. Hence the tension between the Bhumiputra (Muhajirs) and Pardesi Pashtuns.
Queer, in fairness you lot have the same problem in Mumbai, with the tensions between North Indians and Marathis.
You obviously don't know the history of Karachi. Karachi was a major port and economic center long before and any 'muhajirs' arrived. That was the main reason mohajirs went there.
Karachi is in Sindh, and always has been.
To say it belongs to muahjirs is like saying Bradford, England belongs to the desis who are the majority in that city.