I've used the Hirst art molds for a long time - around three years or so. I ordered from him when he had a grand total of three molds.
My number one piece of advice is to use dental plaster. It is absolutely worth it.
Number two, buy a food dehydrator to dry bricks.
Number three, just keep casting bricks for a few days before you start building - it can get frustrating to dive right in when you do not have enough bricks.
Number four, I use tacky glue and I like it a lot.
Number five, I use linoleum tile as a base for most of my projects and glue felt to the bottom of this.
I have a few pictures of things I have built with my hirst arts molds in my story hour. The current episode has a sewer I made from Hirst blocks and a few other things.
Good luck, and let us know how it goes!