Is it allowed to pray salaat in jammat with other people for a namaaz that has been qadha ie can we pray fajr in jammat at zuhr time and does it have the same reward as normal jamaat prayer?

You can’t pray your qadha fajr namaz with Jamat of Zohr namaz if you have not prayed Zohr already.
Yes if your Zohr is Qasr and you finished your 2 Rakat Qasr namaz with Zohr Jamat namaz then in 3rd Rakat of Zohr with Imam you can pray your 2 Rakats of Qadha of fajr namaz But from beginning you can’t start your fajr qadha namaz with Jamat of Zohr namaz.