I looked back at the crowd level for this last Friday (the night we were at DCA when the Halloween party was going on) and DCA was a 4 that day. CLs only measure the wait times until 5pm so that doesn’t consider the non-Halloween party guests with park hoppers that crowd once kicked out of DL.
When we arrived in DCA ~6pm Little Mermaid (typically only a 10-15 min maybe 25 min wait on the busiest of days) was 45 min posted. The Silly Symphony Swings (that I have never seen above 15 min) were 30 min posted wait. Every single food line was 30-60 min & mobile ordering was pushed out 20-30 min most places too. The very wide walkways in DCA were also completely flooded all the way from the entrance up the main path past & inside Carsland all the way to Paradise Gardens.
I have only ever seen the walkways that crowded one other time in DCA & that was on a Dapper Day that got crazy busy at night in both parks (DL closed to capacity for a bit right around when F! & FWs were starting cuz Main St was so crowded and so there were many people flooding into DCA instead). The CL was recorded as a 5 in DCA that day but again, that is only reflective of the wait times until 5pm. But the recent night where we did the Halloween party night felt even crazier than that Dapper Days night.
That being said, you can still manage your way in DCA on a Halloween party night, I would just recommend planning ahead of time to book your FPs for night, plan to eat early or have an ADR or have a late snack to keep you going until you leave for the hotel & set your expectation at a low level in terms of getting a lot done (ie enjoy more atmospheric, walk through attractions like Animation Building, enjoying Carsland neon lights & ambiance, Pixar Pier Preview at Blue Sky Cellar if open at night, not entirely sure on if it is).