Hi,
The client needs to be downloaded as it is the only way you can handle the content as of today.
The client is lightweight, around 400MB. Then you can select what content you want to download, and even work centralized with the cloud server getting only the content of a changeset (a commit in Git's jargon) to avoid downloading the whole repo.
And finally, yes, you can use it with the command line tool: https://www.plasticscm.com/documentation/cli/plastic-scm-version-control-cli-guide
Best,
Héber.