You could create a “Platform” blueprint actor, making sure that you take into special account the length of those Platforms if you need to make sure that they spawn perfectly next to eachother (you would use these length values to offset the spawn position of the platforms if they are of differing sizes).
In the platform blueprint you could define “spawnpoints” for your random objects, as you spawn each platform you spawn the objects for that platform then move onto spawning the next platform etc etc.
If you need a visual example let me know and i could hash one out for you.