The beach sentences are fine either way. Both sentences mean the same thing. The first e-mails sentence is wrong but this has nothing to do with order - it's about the sense of the sentence. The first e-mail sentence and the second one do not mean the same thing.. think about the sense of this - Consider it with these sentences: "Can you send me a letter about the dog?" "Can send me a dog about the letter?" What is the difference between these sentences? When you've answered the question you will see why "Can you send me the project about an e-mail?" is not the same as "Can you send me an e-mail about the project?"

Will the girls go to the beach with us? = This is absolutely fine. You can put 'with us' at the end. Can you send me the project about an Email = Grammatically, this is OK, but it doesn't make much sense. In the real world, it would be obvious that there's a project which both people know about, and the speaker wants to receive an email about this.