There is nothing wrong for you to have a nikaah done first, while postponing the ceremonies of rukhsati and waleemah to be performed later-as long as you do so consensually by agreement of both parties. There is no harm also if men and women sit in the same hall while they observe the ethics of Islamic interaction between males and females. In Islam, segregation is not a requirement; men and women used to interact in the society without any partitions; they were working together even in the battlefields. We read in the sources that women such as Aishah, the wife of the Prophet (peace be upon him, Umm Sulaym and others used to serve the wounded soldiers. Therefore, there is nothing for men and women in sitting in the same hall as long as they observe the Islamic guidelines of interaction and avoid indiscriminate mixing and mingling.
For details on the issue of nikaah and waleemah, please consult my answer linked below.