If I add mint to the milk and cream at the start of a cream cheese recipe will affect the cream cheese?

As a rule, when you are making cheese you don't add in other things with the cultures because it increases the chances you will introduce hostile bacteria. Mint also has antimicrobial properties that may interfere or inhibit your culture. When you are at the curd stage you can add in flavorings. They also highly suggest that you use dry herbs and spices as opposed to fresh, for the same bacterial contamination concerns. As cream cheese is a fresh cheese, you wouldn't want to chance any foreign bacteria getting into it, so I would suggest you stick to using extract as your flavoring to get the best flavor. And add it once you are ready to drain the curds.

Not a cheese maker, but I have tried infusing a milk/cream mixture with fresh mint for an ice cream recipe, and I can't say it was very effective; extract might work better for you if you want a strong mint flavor.