I replaced my alternator yesterday. My last one appeared to have a bad voltage regulator (shimmering headlights, etc...).
However, my idle was almost always steady despite the faults. Very minor fluctuation if any.
With the new alternator installed (well not new, but a tested working pull anyway) the idle is jumping around a bit. Not bad mind you, mostly between the 650 and 800 rpm range.
My hunch is that maybe the ECU is adjusting to the new alternator. But are there other reasons for the idle fluctuation, such as an overtightened belt? Should a good alternator exhibit solid idle from the get-go, or is there an adjustment period?

 

when I replaced my altenator my idle was still pretty steady. I dont believe that there should be a break in period...perhaps like you said a belt is too tight, maybe a bad ground, or maybe even the batery is going bad also (happened to me and i had a crazy idle)...

 

Even though I wrote my first comment above early this morning, I hadn't started the car since I installed yesteday. So my initial post was based on my idle at that time.
Having said that, maybe there actually was some break-in period, because today it was perfectly steady.

 

U still have intermittent air leak somewhere... cracked rubber hoses 'live' following temp.

Did you install extra gnd wire from alt to chassis gnd? Check out the voltage at ECU, is it about the same as at battery (and steady?).