Not all mini simmons are the same.  Get one from someone who shapes one fairly regularly.  I've seen some hideous ones from guys who don't make them regularly.

I'm 5'11 and about 155.  

I had a quad version from Rob Lion that was a really great board.  It was 5'4" x 21.5 or so and 2 5/8.  I used it as my dedicated small wave board / longboard-beater.  Great board.  

Wanted to scale down just a bit on the next one.  Rob was out of town at the time; so, went with Larry Mayo.

Got the Tug remix quad at 5'3" a little over 21" by 2.5."  This one is just as good (maybe better) than the first -- didn't lose any of the glide or paddling in the minor downsize.  I could probably go even shorter but like the 5'3" for the really small stuff and paddling.  When I surf this board in small waves, I often have people ask me what I'm riding and comment on how good it looks like it's going.  I'm a fairly average surfer; it's just the right board for the job.

For reference, my buddy who is an inch or two taller and your weight got one very similar to my Rob Lion but an inch bigger; he loved my 5'4" and got one for himself; he's on the gulf coast and went with Gum surfboards.  I've ridden that board a few times, and it's a great board too.

At your height/weight, I'd go with something from Larry in similar dims to my first 5'3" but might add a touch of thickness since you weigh a bit more.  That way, you get the glide and flow but can still turn it easilly.

Also recently got a slimmed down version of the tug at 5'3" x 19.75" x 2 3/8" ( I think those are the dims).  And that one is really fun too but obviously not as good in tiny waves.

And yes, they might make transitioning back to your normal shortboard difficult; but that's because these are just so much fun, and the easy speed is addicting.