If I understand "fast charge," it is still up to you to determine the charge rate by setting charge amps.  Most chargers have a fast charge cycle and it is a faster cut off of the slow fill at the end.  Many chargers go to C/20 at the end, and fast is usually C/10.  Some chargers will allow you to set the finish at C/5 or faster.

You can finish the fast charge and then use a cell checker and see the voltage, or % charge if it has that feature.  You can also use fast charge, then wait a few minutes and put the battery back on at regular charge and see what it takes to finish.  It isn't a large amount, unless you are already up against the 80% use wall, then it may seem large :)  If you don't need a full charge, it is a great way to speed up things up and may be better for the battery - use the same rate as before and save the time on the finish charge and don't cram in every possible electron.  If the charger fast charge say stops at 4.19 when rested, though set for 4.20 and fast charge, the fast charge should be faster than setting for 4.19 and using the regular charge with slow finish.

Yes, you can parallel charge in most (probably all) charge settings.  The charger does not know what is on the board, for example a 10,800 mah battery or 4 parallel 2700s unless yours has a feature to tell it.  Powerlab asks the number of parallel batteries and then does the math so that you see the charge/battery instead of the total of the lead.

it just stops charging sooner and that's how it does it faster?

Thanks,
MM

That is simplified English, stopping sooner makes it faster.  They were trying to help you understand why/how it is faster.  Try the charging suggested above and watch a cycle and you may understand the difference between faster by short charging to a lower voltage and stopping a bit sooner, versus by a faster cycle termination C/5 versus C/10, and some chargers or cycles at C/20.