Pietersen was appointed last week and joined the coaching staff ahead of the series against Sri Lanka, with his involvement extending through a busy period culminating in next year's one-day international World Cup.
"He's obviously got an incredible pedigree as a player and a lot of guys grew up idolising Kev. He's brought a ton of energy to the setup within the last 24 hours," McCullum told reporters on Monday.
"I've mentioned to him that I don't want him to be buried in one specific skillset. I want him to look across the environment and challenge me privately around ways we can improve or things we can do differently."
New Zealander McCullum was sacked as England's test coach in July, with his tenure marred by off-field controversies involving high-profile players like Ben Stokes, Harry Brook and Gus Atkinson.
Fast bowler Brydon Carse was then dropped from the test squad during their recent 3-0 series win over Pakistan after an incident outside a nightclub.
McCullum struck a firm tone on conduct, saying he would no longer select players who did not maintain professional standards.
"There is a lot of talent around - if you're not prepared to do that, there are other guys who will be given those opportunities," he added.
"As far as Carsey is concerned, it's important for Carsey he gets his house in order. We know he's an excellent cricketer who has done well when he's played for England."
England host Sri Lanka in a three-match T20 series beginning later on Tuesday, before taking them on in three ODIs.