If I have a male friend and a female friend, do I have amikoj or geamikoj?
You can use both, but if you want to emphasize for a specific reason that there are two genders, you can use geamikoj.
Is it really necessary to j on the end of mia? Isn't the j on the end of amiko enough to show there are more than one friend. To me it just makes the word mia look messy for no benefit.