How to mergemine all SHA-256 altcoins? Let's say I implemented AuxPOW to all of the alt coin wallet.
Would it let me merge mine them with Bitcoin or the developer would have to implement them?
The Altcoin blockchain network itself needs to accept AuxPOW. Peers on the network which don't have a node which understands AuxPOW (so, all of the peers on any network) will reject your merge-mined blocks. 

Effectively, you'll be creating your own private fork of the Altcoin blockchain which allows AuxPOW diverging from the original blockchain at the first AuxPOW block, and you'll be the only miner. The developer(s) need to implement AuxPOW, and the community needs to adopt the update. AuxPOW solutions aren't valid on a blockchain network which doesn't acknowledge AuxPOW.