For a mill you need a vise. best have a couple of them. For a mill like yours 2 tooling vices, one flat vice and 1 normal mill vice could do almost all jobs.

Then you will need a kit of hold down clamps. Additional aluminum plate with same T slot will be great.

20x20cm 10mm or thicker could be an ideal tool plate. 6mm or 1/4 screws work best IMO. Spaced holes say 1 inch