Coil-overs I can't help as I have never felt the need even on my V8 Fiero.  However, the one thing you NEED to do, put poly or solid mounts in the cradle as well as do the entire rear suspension in poly bushings.  Then make sure your motor mounts are good and solid.

      Just the stock V6 Fiero engine could cause cradle or control arm twisting, when you add more power from a better engine everything gets amplified. 

Oooh boy, suspension discussion, my speciality.

First things first, Koni only makes the struts (Which means your spring rate would remain the same) and even those you'll have to find someone with leftover stock, as they stopped production a little while back.

Coilovers, if need be, can also be adapted to the stock suspension ( http://www.fiero.nl/forum/Forum2/HTML/046456.html - Here's a link to the Fiero , although this applies to literally any MacPherson or Modified MacPherson suspension set-up). Be advised, however, that it can be difficult to find optimum settings with a coilover set-up on the stock springs. Progressive springs (What we have) will take any linear, easy to solve system and beat it senseless with a complexity bat, making simulators and general knowledge useless.

Also, to tie the two previous paragraphs together, it is always best to sort've "match" the shock rates and spring rates. For this reason, the Koni's would be fantastic - if you can find them, of course. As you dial in more spring stiffness (Say, for a spirited run, or for racing), don't forget to stiffen the shocks to match. Likewise, when running a more comfortable spring rate (Daily driving), harsh shock stiffness will negatively impact both comfort and performance. Something to bare in mind when considering coil-overs.

Here is the main question...how will you drive your car, will you autox, track day, or drive the streets hard like Tristan

I rebuilt my suspension with poly, etc...but went with KYB's, they do just fine...need to ask also on ride comfort

Now my car I just race with...but when on the street, comfortable

Koni's aren't any different than KYB's if you never intend to adjust them - stick with the KYB's if you don't. However, as I stated, if you want to go through the effort of coilovers, you're already in for an adjustable suspension, the Koni's will allow you to take advantage of that.

As for the springs, if you're making a coilover set-up, you needn't buy Fiero springs at all - simply buy a set of springs with an inside diameter that matches your coilover perch outside diameter (Should be between 2.25 and 2.50"). In buying "universal" springs you can get the prices waaay down.