Noob IC questions:
Is the down throw downair cg timing always the same or does it depend on anything like weight/percent? Is it possible to change the time or height of the downair to deal with a marth DI'ing up and away for example?
Also I'm having trouble learning the downthrow fullhop downair cg any advice on how you guys do it?
Down throw is affected by the weight of the character you are throwing, yes. The heavier the character is, the longer it takes for the throw to end. This is also the case with back throw, where up throw and forward throw are not affected.
I'm not sure about the exact numbers, but it goes something like:
Puff - really fast throw
Fox, Falco - fast throw
Marth, Sheik, Peach, Doc/Luigi - slower throw
Falcon, Samus, Ganon - really slow throw
Fox being a bit faster then Falco, Marth a bit faster then the Marios etc. This is just what I know from experience. You can probably look it up somewhere.
You could say that It is harder to do the down-throw-dair on lighter characters, as you have to do the dair faster. If you are used to the weight on e.g. Fox/Falco then you might find it harder to do on more heavy characters (this used to happen to me).
The procent doesn't really matter that much. The higher procent, the smaller becomes the window for your dair to work in (I hope that made any sense), but when speaking about the d-throw-dair CG, it doesn't change a lot compared other stuff like Hand offs where higher procent really makes the regrab harder.
When doing the CG you should either try to time it so you do the dair as soon as you leave the ground, or you can time it so you do the dair in the very end of your jump (right before you hit the ground again.) Both ways should work. Personally I use use the first way because by doing it this way you can L-cancel your dair with the same action as you do your regrab (that being pressing the Z button. You can L-cancel with this as well as R/L.). I believe Chu does it the other way.
About Marth DI'ing. The d-throw-dair CG is very escapable, so if your opponent knows what he is doing, you should not be able to get the regrab. However, if you know how your opponent DI you can use it for your advantage. If you know your Marth player DIs up and away every time, then do a reverse d-throw-dair and follow it by with something like F-smash. If a Fox/Falco player DIs down and away every time, then abuse their DI and do a down-smash or f-smash to send them off the stage. Abusing your opponent's DI is something any IC player should do.
I'm not sure what you mean with "downthrow fullhop downair cg". You should always shothop downair unless you're doing a reverse CG (and even then, it is possible to do the reverse with a shorthop which has great applications in some situations). If you talk about the fullhop reverse CG then practice it on Ganon/Falcon first (it is against these two it really works anyway). Press down for the down-throw and run towards your opponent just afterwards with Nana. Do you fullhop and do the dair just after the throw animation ends. If the dair sends Ganon/Falcon back to Popo, then you're doing it right. If it send him away, you're not jumping far enough behind him. If the dair only does damage and no knockback, then you're doing it too fast.
I hope some of it made any sense. I'm kind of tired.
Besides, you probably knew some/most of it, but what ever.