Bro, this is just my personal advice. The stable of the car really depends on the momentum and gravity force that made by the car during driving. Regardless what size of the rim you use, since the gravity force is in correct manner, you won't get any disturbance movement.
To make sure your car is in ideally stable condition, you can try this:
1) If you use 18" sport rim, try to use dual condition tire (wet and dry), by this u can eliminate the goyang condition during both time (dry and raining)
2) Use the correct absorber type, different absorber type, different specialty.
3) Lowering the car it's can really helpful, because you push the gravity force close to the earth, minimize the air obstruction during speeding, u will feel ur car like melekat atas jalan (F1 proved it!). Furthermore, less obstruction/friction, less fuel consumption!
4) Try to install strut bar for both front and rear absorber, this will help because during you taking the cornering/turn, your momentum direction will change and instead of the momentum push only one side of your car (left or right), this strut bar will distribute the gravity force to the other side (right or left), so you'll feel more comfortable during cornering. But, for sure, on the normal road, it'll be shaking a bit since the both side of the car (right and left) always sharing the movement.
Hopefully this tips could help. :top::top::top: