I banged my head brutally against a rock for several days to get this-
Collision Shapes supporting any shape you could want- as long as it's a convex hull.
Shapes slide naturally against each other just as a natural result of the collision code.
Colliding several (try 70) rectangles together-
Now my brain is like mashed potatoes. WOOOO!
The method I used is Separating Axis Theorem that takes into account object velocity.
It was NOT trivial to figure out the separating vector AND distance and a lot of tutorials I looked for did NOT help or include this.
Next I have to optimize this by spatially partitioning objects/their collision so it isn't naively checking every shape against EVERY other shape EVERY frame.
This would allow maybe several hundred objects (maybe even > 1000) on the screen physically colliding if I should wish.