So im pretty new to Orks and I have a few questions as I want to run a specific list:
1) If I am using the IA:8 Dreadmob army list. Can I take wargear from Codex Orks and Waaaggh Ghazkull? Specifically Da Finkin Kap and MFF
In general when using rules from across multiple books between GW proper (C:Orks, & Ghazkull) and ForgeWorld (IA:8) assume that things cannot be mixed - if you want to use the dread mob rule set, you must pay the points and items as described and allowed in IA8, and do so unless the book itself mentions to use the ork codex.
Also do note that IA8 is a rather old book now (I can't remember if they ever did a reprint of it) and is probably outdated for a lot of things.
So in short the answer is "No", however if you discuss with your friends and house rule it then you could - if you were planning on attending some sort of event then do not expect any such considerations.
2) Meka Dreads use their Warlord trait to join a squad of Deff Dreads, if I run 2 Meka Dreads can both join squads or not?
As I am not an ork player I would assume "no" without being able to read the rule - some times it states that it can or cannot with statements such as "a single ____ may be taken as a _____ instead".
3) For Buzzgobs Kustom Stompa. Which ruleset should I use?
If possible use the most recent ruleset provided, if there are multiple references to it, then either use the book that you are using to build a list from. Do note that whichever book *assuming its printed in Ghazgull & IA* you purchase from it uses that books rules - even if it is either more costly, or worse.
4) Can I use the Great Waaagh or Green tide FoC with the Dreadmob army?
I would so "no" once again, mostly because IA8 was written long ago where formations were only available in Apocalypse, so unless FW has produced a FAQ (didn't see anything on their website) addressing any such questions then my default answer is "use what is provided in the book - do not mix books" for all things used in your questions.
Many Thanks.