Can the latest version of Fluentd, only work with ES-6, or also with ElasticSearch-2?

Can the latest version of Fluentd, only work with ES-6, or also with ElasticSearch-2?

Background:
We want to know if we can temporarily build a new ELK-6 stack, update fluentd and have updated fluend send all data also both ES-6 and ES-2 at the same time.

Also see Simplest upgrade data from ElasticSearch 2 to ElasticSearch 6? for more details.

as i know . es 6 is better than es 2.
thanks.