Can i use NEST version 2.X as a client if the Elastic Search is still 1.X version ?
that's unlikely to work given that the query syntax changed with Elasticsearch 2.x. So although some types of requests may work you will not be able to take full advantage of it.
No, NEST 2.x is not supported as compatible with 1.x; check out the compatibility matrix for more details.
You may find that many requests are compatible, but there are known breaking changes that will result in thrown exceptions, for example, the way that errors are returned in 1.x will break NEST 2.x, so not recommended or supported.