Hi there! It's funny to read your post since our situations are so similar: I also have been with my husband for 7 years and my girlfriend for 4 months.
Firstly, I think it would be useful for you to think about what it is you want? If everybody was totally cool with any arrangement, what would you like to do? What kind of a relationship do you wish to have with Devo? Do you wish for her to become a part of the family and be on an equal standing with Fish? Or are you looking for a secondary type relationship? What's she looking for? I think it is reasonable of her to want to spend at least some of the nights with you when she comes to visit.
Do you know why it is that Fish doesn't want you and Devo to share a bed when he's there too? Is the issue about him feeling bad: jealous, hurt, neglected? Or is it that he feels it's his "right" since you're married, and he doesn't want to share you?
If it's the first reason I'm sure there is a lot of help available to the situation. There's a lot of info here on this forum about jealousy and working through it.
If it's the second reason, him feeling he has the right to your time since he's the one married to you, how do you feel about that? Are you OK with Devo getting "the leftovers" of your time, i.e. only getting to spend the night with you when Fish isn't there? Or do you want something more (and does she)? If so, you need to tell him that and see, if you can come to an agreement.