We would like to edit Columns for example in Peers to add or remove columns, add Client column

From Transfers window, click on Layout, then on Select Columns.

Thank you very much, I would love if you add the leech column to see who's downloading

You are welcome. You can see who is downloading, by clicking on Peers column. By the way, I'm not the developer of Tixati, so I can't add or change features.