So, looking good so far. I've left a few comments, hope they're useful. I'd like to answer those other questions I couldn't answer back there, right here.
Do you think I should go back and forth between past and present or talk about each all at once?
It's ok the back and forth, I think that'd give your narrative an interesting touch, so your storytelling would be everything but simple, which is good.
Do you think this will be detrimental to my story?
I do believe all characters in a story have an effect on it, regardless of how much they appear. One single encounter with one single character may be the decisive of how the story is going to develop. So, rather than being detrimental, it might as well be beneficial.
Should this be science fiction or slice of life?
I think Sci-fi suits it better.
Could this be written in a diary form?
You can write it as you please, hon. That's the wonderous thing about storytelling, there are no rules.