We had 80-foot-high Tulip trees removed from our property when we bought it, because of the liability of limbs hitting the house. No way would I plant that there. They can grow 2-6 feet a year, depending on where you live.

Are you seeking shade? Shade for kids? or Shade for your house? Or is this for a screen? What is the intent?

Maybe keep the pin oak but transplant more in center and back further?

The question is whether future occupants of your home will keep the trees.....more than likely not since most people don't have the same interest or appreciation for trees. I would suggest digging them up and donating them to a park or cemetery where they will grow large and live a longer lifespan. If you insist on having trees, I would suggest smaller growing trees like dogwoods, redbuds, fringe trees, and etc.

That yard does not look very small to me. One could conceivably replant them(at least one of them) in spots where they won't tower over the neighbors' yards, which is what they will do if kept where they are. Is that a park behind the yard? If so, I don't think placing the pin oak at the back of the yard and the tulip tree toward the front/center of the yard is a bad idea, assuming lots of shade is what you're after. Of course there is that other small tree at the back right to consider.

The yard isn’t tiny, but it is only slightly larger than a typical suburban back yard. We definitely want shade, we get relentless sun back there and it makes it hard for grass to grow. Behind our fence is a ditch and then begins the yards of the neighbors behind us, not a park unfortunately. We do love the idea of looking out our windows and seeing trees as opposed to all the houses behind us (we are on the top of a hill overlooking a large neighborhood.. we see a lot of brown siding out our window right now).

I feel like the pin oak is in an okay spot to not interfere with our neighbors, but the tulip will definitely reach our neighbor’s deck on that side. So I guess it’s the one that needs to go. Makes me really sad because I LOVE that kind of tree, but I know it does get big.

The small tree at the back is a lapins cherry, we love it and it’s not interfering with anything so no plans to do anything with it.

I think it's fine, but big trees in confined areas don't bother me.

Pin oaks can send branches down toward the ground; limb it up, and the 'new' lower branches left higher up then angle down. Not what I'd plan in a fairly small yard, given that it grows fast, gets huge and has a large spread that tries to reach low. I like oaks, but if I wanted one, I'd replace it with a different species.

Yes.

To not end up with nearly all of one half of its top overhanging a neighbor a large growing tree would have to be planted right in the center of your portion.

I’m very impressed that you’re giving this some thought now. Many people make decisions based in the cutesy-wootsey plant/tree, with no consideration to growth 20 years from now. I’d keep pin oak only.

Thanks for all the advice. I decided to keep the poplar, but moved it to the back of the yard. It’s hard to tell the real scale from my crappy pano photo, but how it looks in real life (now that I’ve replanted) is leaving me confident there is enough room for everything to grow without affecting our eventual goal for the yard and without bothering our neighbors.