Mshightopp,
Welcome!!
I applaud you for your efforts to support your SO.
My SO is also DID, and a few different ideas come to mind.
When I "married" my Q I made them all agree to marriage, that being said...I do not have any sort of sexual relationship with the littles (revictimizing) however, I love all my Q's alters and most of the adults have a sexual relationship with me if they desire. When our relationship was new, we discussed the sexual aspects of having more than one sexual relationship, if you will, and decided as long as things were consentual it was ok.
One alter was very interested in persuing a sexual relationship with me, but he was only 16 (HE TRIED TO TRICK ME THAT HE WAS ANOTHER ALTER SEVERAL TIMES). I politely informed him that it wasn't proper for me to have a sexual relationship with a 16 year old, he quickly grew up...lol.
Each of our relationships develops as an individual relationship, we start out as friends and things develop naturally from there.
With the littles I have more of a mothering/friendship role.

You said the alter who is coming up is a protector, one of Q's protectors will defend if there is a percieved threat...it goes like this. Eliot and I are arguing about something...suddenly there is Shell...in my face yelling....Shell percieved I was a threat so he came to defend eliot.

As far as the second part of your question about journaling, provide a notebook, pen/pencil, maybe write in the fron what the purpose of the journal is. like "This book can be used by all of us to communicate with each other."
We had starwars word magnets on our fridge and a new alter used the magnets to write things before he was comfortable being around.

Hope this helps,

Its_just_me