what source to choose in electrum when trying to recieve a coin payment from cex.io the options are
( bitpay , bitstamp , bitcoin average , coinbase , okcoin , localbitcoins , coincap , coin desk , coin gecko)
...
but my question is does the source matter at all if i have my fiat currency usd
The short answer is:
NO, it does not matter.
The long answer is:
The "source" value that you select in Electrum is just a
display feature. It does not impact on how much
BTC you will receive. THAT is defined by the exchange you use for
purchasing your BTC in the first place.
Electrum is NOT an exchange. It is just a software wallet that can hold your BTC private keys (or LTC if you choose to download and use the unrelated Electrum-LTC software that is NOT developed nor endorsed by the Electrum developers).
The "source" value simply queries whatever service you select for
their current BTC-$$$ exchange rate so it can display a $$$ value in Electrum. It is pretty much meaningless other than to give you a general idea of the value of your BTC holdings. All the value in your Electrum wallet is actually stored in BTC and is only really worth whatever you can trade BTC for on whatever "exchange" you choose to use to convert it (ie. web exchange, localbitcoins, P2P trading etc).
Changing that "source" will not affect your ability to receive BTC from ANY source you choose (so you can select Source X and receive coins from Source Y etc).