Q: Is it ok to charge your iphone 6 in any power bank?

You should use the charger that came with it. Non-Apple chargers will work provided they are rated at 12 watts and use USB connectors.

Yes, but it's best to use Apple chargers.

The charging "smarts" that protect the battery and make it safe to use different chargers are actually in the battery, not the chargers or the cables.

If they have the same wattage or greater, then you should be fine.