I'm new to JMS, and I have some questions about what's possible with it.
We're operating in a Plesk environment, which I realize complicates things. I have some questions about that, too, but I'll save those for later.

I've been able to create new slave sites, for domains controlled by plesk, by either just redirecting the domain, or by adding httpd.conf lines to change their base directories.

Here's what I'd like to do: create slave sites, from a template, and have them use their own database, and have image and media folders in a completely different place in the directory structures. (Not in the /joomla/multisites subdirectories, because ideally the customers would have ftp access to their directories.)

I can't tell if it's possible at all. So: in creating sites from a template, is it possible to create a new database for the site, or does it have to be a new set of tables? If it is possible, how do you do it?

I want to be able to either use templates, or not, to create new slave sites. As far as I can tell, if any templates exist, it's not possible to create sites without a template. Is that correct?

If I'm creating a site without a template, is it possible to specify different locations for any directories other than 'themes'? Because those options don't seem to appear unless there's a template.

Thanks in advance for your help.

If I have well understood, you are working on plesk and have been able to experiment the plesk configuration in 2 wasys with success
- You have modified the httpd.conf to specify the appropriate documentroot
- You have used the JMS facility to create Symbolic Link into the directory provided by Plesk.

I suppose the Symbolic Link is therefore allowed.
I am not sure because one of your question let me suppose that it is not allowed.

With JMS you can create slave site in different ways
- Create fresh slave sites from the back-end using the Joomla Installation that allow specify a new DB and table prefix
- Create slaves sites based on rules defined in a "website template". This allow to replicate a website into the same DB than the slave site that is used as template.
You can define the "default" new table prefix that must be used to create the new slave site. This table prefix is either used when creating slave site from the front-end or is the default value that is used when creating a slave site from the back-end.

So you can create slave sites from a template into the SAME DB than the website that is defined in the template.
You can only specify new image and media folder when you replicate a website because those parameters are saved in the DB.
In general, I recommend:
Deployment folder = {site_dir}
media folder = images
image folder = images/stories

In your case, as you want to have a specific deployment directory, you should use another deplyoyment folder.
You can specify any directory.
You just have to create the directory before to use it in JMS.
Therefore, you can create a domain or a sub-domain in plesk and use the directory created by Plesk.

Remark the JMS /multisites directory and children directory will also be present because they are used to save the JMS configuration.
All the other information will be deployed in the specified "deployment folder" that is a functionality only available when Symbolic Link is allowed.

YES you can create website into another DB in using a fresh Joomla installation.
You can use the tutorial step 9 procedure and provide another deployment folder.
The deployment folder must exists.

YES you can replicate website using rules defined in a "website template" but in this case it will be replicated in the same DB using a new set of tables.
See tutorial step 13 that show an example when used to create website from the front-end. This procedure can also be used to create slave sites from the back-end.

You can also specify a new "themes folder".
This parameter is always avaiable even when there is no "website template".
I just recommend to keep the same name than joomla and use that absolute path "{site_dir}/templates" or "{deploy_dir}/templates"
In your case I suppose the best is "{deploy_dir}/templates" if you want to have a specific directory different of {site_dir}

So in summary:
- You can create fresh slave site using Joomla Installation without specifying a "website template". See the example in tutorial step 9
- You can create new slave site using a "website template" that consists in the replication of an existing website. In this case the replication is performed in the same DB using different table prefix (set of tables).
- You can specify another deployment directory and the directory must be present before to declaring it into JMS.
- You can specify another "themes folder" and the "website template" is not required to have access to this functinality. It is just recommanded to use the relative path {deploy_dir}/templates in your case.