Ok, let's take a look here:
You said you had the timing belt tensioner and idler replaced already. I think you're confusing the timing belt and the serpentine belt (which is ok, don't feel bad

). Now, if you have a new idler and a tensioner, there should be no reason to replace the tensioner. If, however, there is such a thing as a timing belt tensioner and you need a
serpentine belt tensioner, let me know, I may be able to get you one cheap. I got my idler pulley for $20 and my belt tensioner assembly for $22 from my brother, brand new part.
Since it's only squealing when the AC kicks on, I suspect your A/C Compressor might be the culprit. Is it still under warranty? If so have them check it out. Check out the belt too and make sure it's not cracked or loose; it could be the belt.
If you need ANY more info/help, PM me or post here. I just got done having my A/C Compressor changed out and we replaced the idler pulley and the tensioner, so I've got some good experience with this belt squealing issue; it plauged me for months!