Thanks for the question. In your example you give peppermint and white tea - this would be totally okay to do. Blending these two ingredients - one a herb and the other a tea - is totally fine.
It is when we start mixing multiple herbs together that we can run into adverse reactions with the herbs. (Just like drug interactions).
If you wish to blend herbs to treat multiple problems, please consult a herbalist prior to consuming your mixture of herbs.
It can be dangerous.... also if you have underlying issues like blood pressure etc you would need to let your herbalist know. Certain herbs have effects on a variety of our bodily functions.