Lucene supports reading indexes with segments created with the last major version, so does elasticsearch.
As far as I know, if I upgrade my ES from 5.x to 6.x, then it can read indexes created by 5.x, so after that, I upgrade my ES from 6.x to 7.x, as the first line that I have indicated, it should be work too. am I right?
Yes but only for indices that has been created with ES 6.x or where the indices have been rewritten in ES 6.x (with forcemerge for example).
But the upgrade assistant which is available in X-Pack Basic (free) should tell you that. Note it's not available yet for 6.x as ES 7.x is not ready 
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