I really do like the idea of this episode, and think it could pass as a real episode with some adjustments. I wrote some things that stuck out to me when I read it, and yes I know this isn't completely detailed yet, so a bunch of these probably won't matter, I'm just pointing them out. (Some of these are things that stood out that I liked, some are minor problems, some maybe suggestions and some are just questions.)
-I like the idea of Rarity and Twilight since it's magic and magic, but it should be more apparent why both of them were there.
-I like the idea of Applewood because I love seeing places that are parody's of real cities.
-The wanna be film maker is a good idea, especially since it's so different from the usual actor or actress wanna make it in da big city. So that's great.
-With the potion thing, that might draw some attention, but it could be a great moral about addictive substances and how they ruin peoples lives. A pretty bold moral, but episodes with those morals are usually the best. Also the potion should have a name.
-Okay wait never mind that line above, after reading the whole thing it seems this was not going in the direction I thought. Which is kinda confusing, just slow it down a bit to show what's actually going to happen and that should be fine.
-Pisces like the zodiac sign? Neat, especially since that sign is stereo typically depicted as an imaginative type sign, so him being imagined and being named Pisces was a good touch.
-So they get sucked into the film, while it seems a bit too magic, but it sounds very interesting and I wouldn't mind it. But can you explain how he did it?
-The Pisces thing where he disappears and declares his love and she rejects him, in my mind is perfect, but you really gotta make us really feel what both of them are going through. Like the surprise Shooting is feeling, since apparently she didn't know and the feeling of sadness and heartbreak that Pisces is feeling.
-With shooting going back to Manehatten, it seems a little well not as strong, I was under the impression that she deeply wanted to be in Applewood since Pieces materialized there, maybe instead she should book tickets for her family to visit her.
So overall, I like the plot and most of the characters, and you said this was your first time writing child type stories, well then I gotta say it was pretty darn good, because I can barely write an mlp type story, since there are so many unspoken rules to it. You did a great job and would make a great guest writer for mlp.