The Overflowing Bra

This story has good potential, but there are a few tings that I would consider addressing in your next if not a rewrite of the first story.
Where is the place set?
All we know is that there is a school involved and a street. We weren't told what the place looks like so we can't see the place in our heads.
What do the people look like?
No real indepth mention of the two characters are mentioned, so again we can't picture it ie how big is Melissa's chest before and after the change.
We as the reader want to know things like hair and eye color. Weight and height. Any other facial or visual features that stand out such as dimples when she smiles. 
Too much speech?
Writers love to write speech, but don't underestimate the power of description.
This story just throws us into a universe that we are unfamiliar with and you give the main character exposition. 
Consider a narrator. A third person that we don't see (being you)leading is into the universe and have the main character tell us and other characters what the situation is so that they know as well as us. Just don't repeat yourself with the narrator and the character.
In all good stories there is a balance of talk such as "You've got a bigger chest than most women ever have." to 
Melissa noticed the full curve of the of the alien woman's breast jutting out her tight top.
"Oh, I don't need the simbiont for me," the alien said, touching her charms for emphasis.
Too much talk is a bad thing. Especially if it's page after page of conversation without indication of who is talking. This gets too confusing and turns readers off.
Ignore the person who said it needs too much work, scrap it. Yes it does need work and I for one would love to read it once that work is done.

While there's alot of potential to the story, you jumped around a bit too much to make the story enjoyable. Also, more detail would be good, and please keep track of your plural and singular forms.
Nice touch about the porn star and the school though.

I think I have to go with Darkheart here. The idea is good but I think that perhaps - if you're going to work on it later, that you could try and remove the whole dimensional thing as I've already articulated at the Plume, it seems superflous and overkill and detracts by adding additional elements that are simply confusing.
The idea itself is a good one though and certainly, it would warrant an indepth reworking as a full length piece or with some modification it would be a good start to a thread on the Addventure.

I hope you're not depending on others to finish this story, I can't remember the time that's ever happened.

it needs too much work to be a good story. scrap it.