There is no selection process in the Pixel TV episode.
In that episode, the four featured disguises are all seen in the disguise machine's chambers before being stored in Robbie's trunk.
The disguise machine's chambers contain (right to left):-
Robbie wore: A chef's hat, a white long sleeve cooking shirt, a blue handkerchief, dark white pants, and a black mustache.
|Seson one: Rob U. Blind | Lazycus | The mailbox | Robbie's softball uniform | Sherlock Foams | Sportacus | The Birthday Fairy | Scotty the Scoutmaster | Dr. Rottenstein | Rottenbeard | The Dinosaur | Sir Rottenham of Robbington | The Rotten Kid | The gorilla suit | Soccerbot | Miss Roberta | Fordmil Meansbad | Santa Claus | The purple cow | Giant fruit and veg | Zobby|
|Season two: Johnny B. Badd | A purple bush | The Trash Man | Mayor Meanswell | The Ghost | The Snow Monster | Roberto the Great | The Shaking Cup operator | Rob R. Robley | The Purple Legend | Chef Robert | The gorilla suit (again) | Prof. H.R. Rottennen | The sticker vendor | The construction worker | Bob Wire | The Big Bad Wolf | Robbie's racing outfit | Sully Swingahip|
|Others: The aristocratic dress | The baby | The blue dress | The chicken | The court jester | Cowman | The crusader | Robbie's dressing gown | The nightgown and headdress | The silk clown | The white robe|