I tried to do this in Infopath, and I just don't have the knowledge to get
it working right. I'm hoping that there is an easier way to do this.

I have a document library. (this can be a form library if necessary.)

The following are columns in the library

Title
Revision
Status
Transmittal #
Transmitted by
Transmitted Date
Vendor

Here is what I would like. I need a form for this library, for when a user
clicks "New" the form opens and is automatically numbered. In the form there
should be about 20 drop down list fields so that a user can select documents
that have been uploaded to the library. When a user selects a document in the
DDL, other fields in the form should automatically populate with the appropriate
fields from the document selected. For instance, say I have 4 fields in the
form, labelled FileName, Title, Revision, and Status. The FileName field is a
drop down list allowing a user to select a file from the library. When a user
selects the file, the other fields in the form will be automatically populated
with the appropriate information from the document library. (So far I managed to
accomplish all of this in InfoPath, but it was a hack and I had something like 6
data connections that would need to be updated every time I implemented the form
elsewhere.)
When the form was submitted, or saved, to the document library, I want it to
apply the number generated when the form was created to the Transmittal # field
of the documents that were selected in the DDL of the form. I have searched
around and have found out how to add an item to a list using InfoPath, but not
the answer I am looking for. I would have thought that this was something that
would be fairly straightforward to do, but apparently not. If someone could
please help me out with this, on a one on one basis, I would greatly appreciate
it.