I think the difference between & and / only matters when it's an exchange that matches on relationships rather than solely on characters.
So, say you wanted to request Mara: Daughter of the Nile for Yuletide. Yuletide matches on characters, so whether you wanted shippy or gen stuff for Mara and Sheftu, you would simply request both characters, and then say in your letter that you were happy with either shippy or gen fic. And because Yuletide doesn't match on relationships, all you would be guaranteed of getting is a fic in which both characters appeared — whether it was shippy or gen would be down to the author.
But in Interlibrary Exchange, you have the ability to match on relationships, and participants make offers not just of characters, but of specific relationships. For me personally, this means that I am going to be more explicit about requesting both (using your example) Mara&Sheftu and Mara/Sheftu if I would genuinely be happy with shipfic or gen. There are some fandoms I've nominated where I literally only want shipfic for particular characters, so that's all I'm going to be requesting, meaning that anyone who offers just gen wouldn't match to me. (And I would personally find it confusing if I offered only an & relationship and then the person I got assigned to mentioned shippy prompts in their letter, without having any evidence of having requested the / pairing as well.)
Basically, in answer to your question, I think you need to be as clear as possible in your requests if you really are happy with both gen or shipfic for particular pairings, and request both the & and / version of the relationship. I think that just using the & version is liable to cause confusion for your assigned author.
no subject
So, say you wanted to request Mara: Daughter of the Nile for Yuletide. Yuletide matches on characters, so whether you wanted shippy or gen stuff for Mara and Sheftu, you would simply request both characters, and then say in your letter that you were happy with either shippy or gen fic. And because Yuletide doesn't match on relationships, all you would be guaranteed of getting is a fic in which both characters appeared — whether it was shippy or gen would be down to the author.
But in Interlibrary Exchange, you have the ability to match on relationships, and participants make offers not just of characters, but of specific relationships. For me personally, this means that I am going to be more explicit about requesting both (using your example) Mara&Sheftu and Mara/Sheftu if I would genuinely be happy with shipfic or gen. There are some fandoms I've nominated where I literally only want shipfic for particular characters, so that's all I'm going to be requesting, meaning that anyone who offers just gen wouldn't match to me. (And I would personally find it confusing if I offered only an & relationship and then the person I got assigned to mentioned shippy prompts in their letter, without having any evidence of having requested the / pairing as well.)
Basically, in answer to your question, I think you need to be as clear as possible in your requests if you really are happy with both gen or shipfic for particular pairings, and request both the & and / version of the relationship. I think that just using the & version is liable to cause confusion for your assigned author.