Can we use other databases (MongoDB, MySQL) as data node So that we can keep our existing database as it is.
If you want to think of Elasticsearch as a search accelerator for your application that is fine. It pretty common. Just keep your data where it is and sync documents to Elasticsearch as they change. You are responsible for noticing the change and then sending them to Elasticsearch. The advantage of doing this is that your documents in Elasticsearch don't have to match how you want to store the data in MongoDB or MySQL.