How do I stop the pins on my pincushion costume from drooping?

The problem isn't your pins, it's that polyfill has basicly zero compression resistance, so anything stuck in it just flops over. Hot glue wraps won't fix that because the glue is squishy against fluff no matter how thick it is. What actually works is giving the pins something solid to anchor to inside the shell.

What I'd do: cut a piece of thin plywood or even a couple of layers of that stiff plastic canvas from the craft store into a panel, drill or cut holes where you want the pins, and glue the dowels thru it so they're all connected to each other. Then you can position that panel inside the fleece shell wherever the pins come out, pack polyfill around it for the puffy look, and the stuffing never has to hold anything. Pins anchored to a rigid panel will survive all the walking and bouncing you want. If you don't want to build a panel, the other trick is a dense block zone. Cut a chunk of upholstery-grade firm foam or a piece of EVA floor mat to sit inside the shell at the pin area, poke holes in it slightly smaller than the dowels, and force the pins in. Friction on that stuff is way better than hot glue on a dowel ever was. Fleece is thin enought that nobody will see the block, and you can still fluff polyfill around it so the rest of the costume reads as soft. On length, two feet is a lot. Mine were around 15 inches on a pincushion costume I did and even those cleared door frames by a little, and I had a moment on a bus where I realized I was one turn away from taking out a stranger. The drama is mostly in the ball heads and the angle they stick out, not the length. If you drop to somewhere between a foot and 18 inches you keep the silhouette and quit spearing people. If you keep the long ones, at least sand or cap the tips so the ends are blunt. A blunt tip on a two-footer is annoying to get poked with, a sharp dowel end is a whole different story.

For what it's worth, I went a slightly diferent direction on mine years ago and sewed two crossed strips of stiff webbing into the shoulder seams, then zip-tied the dowels to those, which moves the load onto your shoulders instead of asking the stuffing to do any structural work at all. The pins still wobble a tiny bit at the ball heads, which honestly sells the effect better than rigid rods would. One thing I wish someone had warned me about: practice sitting in your car before the party, because even moderately long pins rotate toward the seatback when you lean forward and it gets claustrophobic fast.

On the length thing, a tape measure is honestly what saved my last big costume. I walked the whole route I'd be wearing it on, my hallway, the parking lot, the venue doorways, and measured the tightest clearance, then sized everything to sit well under the smallest one. Two minutes with a tape beat any guessing I'd done before.

One small thing that helped with the lean on mine: I drilled the mounting holes at a slight outward angle on purpose, so when the pins sag a bit they droop away from me instead of inward toward my face, and the ball heads read fine even tilted.

One thing that helped mine was swapping the dowel diameter. I started with 3/8 inch rods and they bent at the entry point where they flexed against the stuffing, so even a solid anchor underneath still left a floppy lever. Went up to 1/2 inch hardwood dowels and the lean basically disappeared, because the stiffness goes up fast with thickness. Also, glue the foam ball onto a short wooden bead or button before threading it on the rod, so the head sits tight instead of wobbling on the tip. Made that mistake on my first one and every pin head developed its own little nod after one party.