Make sure whatever water is being taken in for filtration, cover up the intake. They make super small protective coverings to allow filtration but it is designed to avoid little babies being sucked up!
I will say, I have a couple of seahorses that seem to like to dive into the output water flow and go for a ride! They do it more than once and it looks very intentional. I have also seen inexperienced ones go into that same flow and they go flying, so I think they learn what they can get away with.
As for the dragonet, I have one in my DT tank, but seahorses in there though.
That said, I could potentially see a dragonet in there with the seahorses if it started off small and realized these are cohabitants. I just do not know though.
My dwarfs love to swim and eat off the bottom, so I could imagine they would interact with the dragonet, whether by choice or not. Maybe introduce one and watch how it behaves. They eat the same thing, super small baby food, so maybe the dragonet looks at a baby seahorse and thinks it is way too big to even consider eating.
It certainly would compete for food, so just making sure everyone is fed well! Fun times for sure!! They are so fun to watch!