As was said, if you add different clowns in a 35 g, be ready for issues. Even if they kinda get along, you are adding the stress element to the environment. I kinda see it as a hostile work environment. Nobody is killing anybody, but everyone is always on edge, which is not healthy.
I also would not recommend adding and engineer goby to a 35g or at least plan on rehoming it down the road. I had one that was about a foot long. I can't imagine that in a smaller tank.
Coralline needs good parameters, lighting etc. It would be tough to predict how long a tiny amount on a shell would take to seed a tank. A lot of LFS
sell small pieces of live rock covered with coralline for around a buck. I would look into getting some of that.
As for costs for special ordering...I've never heard of a LFS
doing that. Every one I've ever been to will just charge whatever they would have normally charged if they had bought it. It does not cost them anything more to order it, if their supplier has it in stock. If it's something rare and they have to find a supplier, I could see them adding the cost to you. Normally, they just add it to their list with everything else. Some stores will ask for a deposit, which is fine.