I went to get a bike fit last week. I have a 54cm allez frame. I'm using an 80mm stem and riding at about 43-44 degrees. Not very aggressive at all. My problem is that I'm not flexible at all, specially in my hamstrings. The fitter recommended a stretching exercise and yoga. My question is, would a smaller frame or maybe 70 mm stem allow me to ride more aggressively (lower than 43*) even with my lack of flexibility?

forget about aggressive position until you can touch your toes without bending your knees. 70mm stem on a 54cm stem is awfully short & is probably quite unstable

An aggressive position is an aggressive position is an aggressive position, period. The way you ride a bike doesn't change between sizes if it's set up steeply. McJimbo's right, it's time to face the music and do all that stretching you've been neglecting to do since you were a kid. As your flexibility increases, you'll notice that you'll be able to ride in the drops for longer, and will be on your way to losing spacers or running a steeper stem. But it's going to take work.

Also, I wonder how your bike is set up as a whole? Even an 80mm stem sounds twitchy as hell...I'm used to using 120's.

As others said, you don't need another frame to get a more agressive position, you just need to be flexible enough to bend over more.

yea that i cant do yet. its only been 3 days of stretching. the fitter told me it took him about 6 months to start noticing the difference. i think it will take me longer though.