[...]
With that being said...can anyone give me advice? My key was generated by coin.mx, was sent to bitcoin before it synced, dumped the key there then swept by greenaddress wallet and blockchain wallet (at the same time) and stayed at greenaddress.
With that being said...can anyone give me advice? My key was generated by coin.mx, was sent to bitcoin before it synced, dumped the key there then swept by greenaddress wallet and blockchain wallet (at the same time) and stayed at greenaddress.
The last one is the one that currently matters. If you trust greenaddress that they generated it with a proper random number generator, then importing that key won't do any harm. Even more so if you will spend from it soon.
Don't reuse the others or any receiving address. Address re-use is considered by some to be a bit of a privacy issue.
But I see importing has its downsides, as if I ever want to spend from that key even just a portion, I'll end up spending all of it.
If you use Bitcoin Core to spend from it, it will look like it spends all of it, but actually the "change" goes to a new address (hidden from your view) in Bitcoin Core. So your balance shown in Bitcoin Core will be as expected, but if you look at your address on a block explorer it will be emtpy.
That is also where some of the confusion about importing comes from. There were cases where people imported a private key from a paper wallet, spent from it and then deleted the wallet.dat file. Since Bitcoin Core sends change to a new address each time those bitcoins in the change address where lost. In the meanwhile there are now other solutions to spend from paper wallets such as Mycelium (Android, iOS) and others, which sent the "change" back to the paper wallet address.
Keeping the above in mind (Bitcoin Core generating addresses in the background), it is recommended that you back up your wallet.dat every 70 transactions or so. That is an advantage of hierarchical deterministic wallets like Electrum or Armory that you only have to do a backup once.