My team is planning to install Sitecore 9. We are using Solr 5.5 version at the moment. Can we use this Solr version? Is this Solr version compatible with sitecore 9 and if we can work with this?
By official documentation, Solr 6.6.1 is compatible with Sitecore 9. This version of Solr 6.6.1 is officially tested and recommended.
You refer to office release notes https://kb.sitecore.net/articles/227897.
The Sitecore 9 initial release had bugs, so the patch version was introduced. Now the latest version is Sitecore 9 update-1.
Sitecore compatibility table shows all compatibility of Sitecore CMS and Sitecore XP with different browsers, operating systems, .NET frameworks, and database servers.