Iron is basically worst for anything. Only reason to use for it is Inertial Dampeners, and/or lack of other materials. So move to Titanium asap, but still, even ship made out of Iron should be able to turn.
Now, if you check Workshop, you can easily see that there is a lot of good-turning ships, so it IS possible, now let's see how they do it.
First. Thruster, especially thrusters are your friends, so you need to know how they work.
(just in case: "thruster pancakes are no more, thrusters are now volume-based).
Thrusters give you more torque (which is what you want to rotate fast) the more lever they have. Now, to gain more lever, you want to place your thruster so that it would be as far from center of your ship as possible. You can get good results even by just placing simple (non-directional) thrusters somewhere far from the center of mass of the ship. Bad rotational speeds can then be covered by gyroscopes (be sure to rotate them to achieve effect on the desired axis of rotation!)
But to really control and imrove particular rotation speeds even on high-mass ships, you need to use directional thrusters.
You get maximum result if you place directional thruster rotated to certain direction (axis) if you place it as far as possible from the line going through the center of mass in the same direction.
I bet this is completely non-understandable, so some more practical examples:
If you want to get more result from the thruster oriented along "up-down" axis, for example, you want to place it far to the side (left-right), in which case it will give you more roll, and/or to the back or front, in which case it will give you more pitch.
Applying general principle to this example, you maximise lever by placing this vertically-oriented thruster as far as possible from the vertical line going through ship's CenterOfMass.
Likewise, to get maximum yaw/roll form "left-right" oriented thruster, you place it further to top/bottom , and/or further to back/front.
And, finally, to get maximum yaw/pitch form "backward-forward" oriented thruster, you place it further to left/right, and/or further to top/bottom.
Hope this somewhat convoluted explanation helps a bit. Good luck!
Fantastic, I get what you're saying, thank you! To get over my paranoia of having my good thrusters blown off in combat, can I put armor over them? Or will the weight on the sides really affect how well they work since it would start to inch them towards center mass?