The simplest approach would be to write:
interaction.content contains "#myhashtag" and
interaction.type == "twitter"
Regarding the "RT" text in retweets, Twitter only recognises a retweet as a 'true retweet' if it was sent by clicking one the the official retweet buttons, and the Tweet itself is unmodified by the user. If you edit the Tweet yourself to say "RT @user .....", this will be treated as a new Tweet.