To install brush downloaded from deviantart - first if file is zipped, you have to unzip it (using for example free iZip app).
Then you can tap "Open in..." (in iZip, Safari, Files etc, it depends where you store the file), and choose Artstudio Pro.
You can also open Brush Editor in Artstudio Pro, tap "Share" icon in top toolbar, and choose "Import ABR/TPL/Artstudio".
To create square brush - the easiest method is to choose any brush from "Basic" group, the go to "Stamp" section, and change Generator from 'Circle' to 'Square'. The other method is to craete fruch from Stamp image: create new empty canvas with Black Background, let's say 100x100 pixels. It is already square shape, so just tap File>Add Preset>Add Brush.