The quick answer is yes. You will not, however, need to concern yourself with External Review as they are simply receiving the documents, nothing more is expected of the process.
To deliver multiple documents you would need to perform a search that returns the documents you want sent, as they are related documents you could perform a search on the related fields value, that should manage to only get related documents. Unless of course you mean they are related in a business sense, in which case grouping them together somewhere and searching for them in that location would result in a similar result. After you have the Variable of documents, simply use this variable as the attachment option in the Email Document step.
An alternative, if you wish to monitor/audit the customer review could be to use a public link. Caution here, as this does expose the document/folder to the outside world.
When you say preform a search are you referring to the find document by meta data? In regards to the related documents I am currently manually attaching the documents as a related document. There is no metadata tied to this documents.
In order for me to know which documents are related I have the reps upload the document in the doc launcher form and then when they submit the workflow there is a step that leaves it up to me to manually attach those documents as a related document.. Is there a better way of doing this? Could/Should I create a folder that will store all the documents being uploaded in the doc launcher into that folder and then reference that folder in the email step?
This is how the rep will upload related documents... then I will attach those documents as related docs.. see below. 
This is how I relate the documents together.. 
Could you review your Workflow variable from Document Generation after a kickoff and see what all values are present in the Documents branch? My expectation is that you will find them all there already for you. If not, could you provide more detail around how you "manually attach those document"? I believe this will give me insight into how you could collect them all.
I manually attach the documents by having a choice step for myself were in the task document view I go to info and hit attach next to related documents. see image below
I usually get the document id of each document, and make a variable with the multiple IDs.
