A dobe would fit the bill just fine. I'm sure ours would happily welcome a tiny kitten or cat or anything else he could consider a playmate.
My parents have 3 dachshunds (2 minis, 1 standard) and T was so very good at playing with their little long haired mini. He had a blast and even though my parents though he was trying to eat him, I knew he was just playing (he puts his mouth on top of the neck to play - does it to our greyhound all the time). He doesn't care about the ducks, but also doesn't go out of his way to avoid plowing them over when he's tearing around outside.
He thinks the horses next door are just huge dogs and doesn't get why I don't want him climbing under their fence to play with them too.
They are loyal. Ours is a shelter dog from Kodiak. He was with us as a foster for 2 weeks before DH got underway for 3 months, so he bonded with me and then with DH when he returned. Here, he shows some protection traits. If somebody comes to the door when I'm home alone, he does NOT want that person here. He lets us know we have visitors when DH is home, but he's not at the same extreme. I have no doubt he'd be there if I needed him to protect me.
I have a greyhound as well (have had 2, DH has had 3 prior to marrying me). Love the breed. My 2 also got along with my parents dogs over the years, but Ryan would grab them if my mom let them out in the yard with the big dogs to run. If it was outside, it was fair game to Ryan. But they may well give chase to anything that catches their eye, so they need to be fenced when not leashed. I do know of people that have had issues with an invisible fence working for their greyhound, 2 that I know of. Others had no issue. Most groups will say no to invisible fencing if you plan on adopting. I've also been on searches for missing greyhounds when their owners let them out to potty with no leash/fence. Jet took herself for a walk when we first moved here while we were working on the fence. Luckily, there were some state guys at our driveway that saw a fawn colored dog heading south on the road when they drove up. DH found her half a mile away snooping around some neighbor's yard. She was 11 and knows she's not supposed to leave, but she wanted to, so went for her own walk. Yes, she has been off leash, but that was in Kodiak and she was always within my line of sight and we'd worked with her for years. If she stopped listening, right back on the leash she went. Here, we fenced the back 4 acres, so she is fenced again.
But neither of my greyhounds has a protective bone in their body.
I love the greyhound breed and we will have more, but I don't think a greyhound would fit what you are looking for. I do think a doberman could easily fit the bill.