do you let them gain weight slowly or quickly.

hi guys i'm just wondering how i should put weight on ralph. i'm giving him extra pellets and extra helping of carrot but just wondering is that enough or should i be doing this slowly. i had also thought of giving him some mix at night time as i know it's fatten, but at the same time he does some mushy poos so don't want to upset his tummy to much. any ideas? i did read about corn and oats but not fussed in giving him that.