Hello Demons are very powerful souls, their power is beyond human comprehension. You can't control a demon to assume a certain form when he/she comes to you (if that's what you meant). The thing you can do is to work efficiently on your astral senses, more you open and empower them, more you'll be able to see your GD in different forms (hearing it's voice, sensing their touch, seeing them etc). Of course, if you're not comfortable with a specific sense, you can politely ask your GD what "form" of communication suits you the best. Again, i don't know if that's what you meant by your question.
You don't choose them, they choose you. It is better to think about how you can become Mightier and closer to the Gods. All the answers are in your Soul, you just need to «look inside».