I was thinking the big/small mode separation would be the difference between being able to clear 1+ times vs being able to clear only if you play near perfectly. Small mode you would go through the game and be able to clear it, but maybe with some sort of thing that makes it just a tiny amount harder each time so it isn't infinite, like you have to place an unmatchable shape for example. I bet some other people also do like being able to clear through it so that is the idea for that mode. I haven't thought through it too much though so it could just be that the shapes are a bit smaller without extra changes, squares and triangles could be about the same.
I'm also surprised the other modes aren't played as much, but a lot of people just come here and want to compete with scores from the original game directly after watching streamers (it's a big streamer game right now) so until streamers play the other modes it will stay like this. I think more dedicated players definitely give them a try. I personally like random a lot, it feels somehow like a rogue type experience, different every time but there is a lot of complexity to think through.
As for adding shapes through the workshop, it likely won't be possible. There are a few failed shapes I tried adding, ovals, stars, corner pieces. I tried a wide range of physics settings and workarounds and still couldn't make these work yet. Ovals were not bad, but get stuck inside each other, corners and stars were cool, but in order to resolve physics under stress the entire box of shapes would spazz out, everything rotating with insane forces to try and depenetrate all the objects. I used various baumgarte scale values, bounciness, tried 2d and 3d with constraints, different collider setups etc. I have a scene I run where it just spawns a ton of objects in an enclosed space to stress test shapes. A lot of shapes you would think should work somehow just haven't so far but I'll continue to try and make more. One type of shape I think I could add and tested a bit is a capsule shape, which is close to rectangle but has round ends. This might be next
(Theoretically there might be a way to have someone provide a path of polygon points and set everything up from that but I don't know if I'll jump at the idea immediately, since few of these collider types end up stable)