Yes you can use the server for other functions. Zimbra installs all the necessary software required to function and so there will be Port conflicts between such common applications as LDAP, web and email services if they are already on the server. The solution in this case is to have the services run on either unique ports or bind to different IP addresses.
Yes, you can have as many Zimbra servers as you require. There is no additional cost for deploying multiple Zimbra servers. To install multiple Zimbra servers, please see the Zimbra multi-server installation guide.
Sure! See this page, Zimbra Single Server Installation.
No, there is no extra cost to deploy additional Zimbra servers. You can have as many Zimbra servers as you require. Licensing is based on the total number of mailboxes in your organisation.