The size of the team doesn't matter. It will fit your need regardless the number of the developers.
For the client interaction, you create an account for them with the right permissions and that's it.
I've been working for about a year now with JIRA and Confluence and I can't belive I didn't use it before.
I looked into the Atlassian tools but they seem too overkill for me, currently the project is still quite small and I just have a couple of clients, and only collaborating with other people occasionally.. But thanks for the tip! Will definitely look into it again when the project has grown bigger.