Probably I'm basing this more on pedigree, but I think Alex Kirilloff is the Twins future at first base for the rest of the decade. I think he will hit as well as Julien and be a better defender. We've only seen glimpses of a healthy AK, but it is pretty good. The injury history is concerning, for sure, and having an adequate backup/platoon partner is prudent. With the current roster, that guy is Miranda, with Julien a possibility if Kirilloff is injured and out for a longish period of time.
It is not optimum to slate Miranda as the RH platoon partner because he's coming off his own injury and poor performance season.
Kyle Farmer absolutely should be getting the majority of his at-bats against left handed pitching, but he's not a first baseman. He's probably the Twins best defensive option at second base and he has both career and recent success against lefties. Fitting him in at second against left handed pitching makes perfect sense as long as he's on the team, however, he can move around and give other players a day off or a chance to DH. I would hope that the manager finds a way to avoid strictly platooning Farmer and Julien.