Pour olive oil into the bottom of the slow cooker.
Place chicken breasts in the slow cooker and sprinkle with garlic salt, garlic powder, and chicken fajita seasoning.
Add the frozen fajita pepper mix and the can of diced tomatoes with juices on top of the chicken.
Pour chicken broth over all ingredients.
Cover and cook on low for 4 to 6 hours, until the chicken is tender and fully cooked.
Remove the lid and shred the chicken using two forks directly in the slow cooker.
Stir the shredded chicken and vegetables to combine with the fajita juices.
Add the heavy cream or milk and shredded cheese, stirring until the sauce is creamy and well combined.
Meanwhile, boil the penne pasta separately according to package instructions, then drain.
Add the cooked pasta to the slow cooker and gently stir to coat it with the sauce and chicken mix.
Serve warm, garnished with chopped parsley or cilantro and extra cheese if desired.