Custom objects are allowed and yes, they follow the same rules. Stick them to the ground*.
Regarding mixing blocks into the ground, that's also allowed as long as they don't create any ugly gaps in the terrain (which I think doesn't happen any longer but I'm not entirely sure about that)
* Technically you can tie objects to blocks, and their height can be smaller than 1. "Stick them to the ground" means that they're in height "ground" and with their bottom touching a surface (i.e. another block).