tx for advice Keenplatypus. I was thinking about wits end from the start. CDR runes ... damn i didnt realize thats only 1 second less. I will make newe build with things u proposed( 3rd kogmaw to sellect.

Ap As Hybrid really nice choise i saw 1 time teemo with it and he was owning kinda save game i belive kog can do the same good 1 +1 keep it on!!! :D

Hello. I like what your going at so far but i have a few concerns.
First, I notice you max your E second. I like to get that last because your slow mainly serves for getting kills and is therefore more suited for late game whereas your Q is universally useful due to the passive attack speed bonus.
Second, CDR on Kog'maw isn't as glamorous as you think it is. The skill with the longest cooldown is his W. While this seems ideal, 17 seconds with 8.1% CDR equals about 1 second reduction. This cooldown also start when the skill is triggered, so it much more like a 7 second cooldown. All of his skills are relatively easy to spam if need be.
Third, I'm not a fan of your last two item choices, in this case Phantom Dancers and Bloodthirster.
The Phantom Dancers mainly promote AD or Crit Champions, whereas Kog'maw would benefit more from just the AS. I recommend Sword of the divine Simply because it has a higher attack speed bonus and helps to counter dodgy champs such as Jax.
The Bloodthrister is more like a snowball item due to resetting after you die. When playing a squisher character these are not as much a good idea unless you are doing well. Additionally Kog'maw wouldn't survive if focused anyways, life steal or not. I recommend Wit's End to replace that. Wit's End gives attack speed, which coupled with the other items may replace the runes and free them up for a better selection, as well as increased MR which is much better especially in dealing with enemy casters.