So... to come back and post on how things happened (as promised), I ended up texting with Dennis. And I let my partner know that I had... he seemed a little taken aback, but I went down an informational road, letting him know that I let Dennis know we were excited to see him, etc. There didn't seem to be a question after that. It came up later on lightly tho, with another situation ('this is why one person should be the point person'), and I let my partner know this is why I'm reading books 'Ethical Slut' and others to get a grasp on how open relationships work best. He acquiesced to that, and allowed for me to share more on the topic, as with the books, I seemingly have more knowledge on the topic now...
In the end (with Dennis), he didn't follow up on our hookup (flaked), but then he followed up a week or so later again with my partner. He might be more into him than me, potentially. We're both 'eh' about it now. So, on to other things:
Now, along the lines of 'rules', I said from the beginning that I'm uncomfortable with my partner having someone over to spend the night for a hookup (we don't live together). He said he would rather have that as a 'suggestion', and not a rule, as he knows younger guys that don't have a car that take public transportation, and it would be more convenient if they spent the night. On one occasion, someone who he keeps mentioning, after a couple times of hooking up, spent the night. I didn't feel comfortable (as promised). My partner got annoyed that I got uncomfortable and said it was my own insecurities (I tend to agree, of course, but to some degree) and that my uncomfortable nature about it takes away from our relationship, as if I'm saying it's not strong enough to overcome such a thing. His interaction with the house-guest is apparently friendly and sexual, but not romantic. Of course there's cuddling and affection, and multiple times of sex along with jokes, laughs, etc... so it's hard for me to wrap my head around, I guess, thinking it's just a 'hookup'. I think the part that is most tough is that I'm sad that he doesn't seem to be concerned that I am uncomfortable ('I'm not feeding into your insecurities', he says). This is all so new to me (both of us) in having an open relationship, and it's been under a year. We've been open from the very start. I feel as though I've made leaps and bounds, and am working hard to try to process all of this stuff and look at why I'm having a tough time with it personally (jealousy/insecurity, etc.) but wish that my partner would be a little more sensitive.