I'm curious why Fluttershy and Rarity are not allowed in the Uncommon Ships. I've combed the site and have found a grand total of 52 stories featuring the couple.

1325716
What!? Seriously? There's that many? Ok...Calm down God...Ok dude. ya gotta send me the link to some-a those in a PM.

1325730 How about all of them in one handy group?

1325716
Fifty-two isn't a terrible number. There's probably more, which just haven't been added to that group.

Still, it's decidedly not enough in general. I love my Flarity.

1325821 If there are, they aren't tagged in a way I can find them. I searched for every romance story featuring Fluttershy and Rarity in which they aren't directly shipped with someone else.

Allow me to answer your question, ahem:

I feel like any ship between the mane 6 gets a default common. That and any ship with Rainbow Dash.

1347211
Not necessarily true.
RariDash is a very uncommon ship, and so are the following-

TaviDash [Octavia x Rainbow Dash]
SunDash (it's so rare, there isn't even a fic about it yet)
LunaDash

-just to name a few at the top of my head. Might make a separate thread about them.

4054668 Raridash has 158, actually. Flarity is slowly climbing, it just topped 100 recently.

4054894
Yikes!
And here I thought it was the other way around.
But still, those other Dash-ships I mentioned are pretty rare (as far as I know), right?

4055067 yes, tavi anything but vinyl is rare. Celestia pretty much anything but twilest and discordia, and luna anything but Tuna. Those are all guarenteed to be rare.

4055432
Yeah, thought so.