I agree that it could just be delayed for capacity. Possibly likely even.
As far as TSL capacity, Mater's Junkyard Jamboree only does 600 per hour and is also dual-sided, so expect Alien Swirling Saucers to do the same.
As far as Slinky Dog Dash is concerned, according to the concept art, the trains will seat 18 per train (9 rows). Also going on the concept art, it looks like 1 train out at a time and 1 train pulling back in for unload/load. I'm going to be kind and say the ride is somewhere around 1:30. So at a 1 minute dispatch (completely guessing), it will have what, a Theoretical hourly capacity of roughly 650 riders per hour? I'm using some guesstimating, but I feel like most of my numbers are pretty on track and I believe I did my math right so if that's the capacity then... ouch. TSMM is technically part of the land though and has an 1800-ish capacity. So the land altogether is right around 3,000 on my numbers.