Does InnoDB allow adding columns to a table with concurrent READ/WRITE of rows?
I have mentioned ALGORITHM=INPLACE in two other posts


Mar 27, 2014 : When tables are locked does MySQL queue queries?
Feb 12, 2015 : How to evolve MySQL schema while maintaining integrity


In the second post, I named some caveats on using online DDL


  The online schema change feature uses the folder mapped in tmpdir. Make sure the copy of the table can fit entirely in tmpdir. For example, if tmpdir is mapped to /tmp and the table you are changing is 7GB, there has to be more than 7GB free in /tmp. If /tmp is not large enough, please consider mounting /tmp on a large volume.


What I have mentioned my seem like overkill to discuss. Notwithstanding, the point I am making, to answer your question, is yes you can do INSERTs and SELECTs

CAUTION: You should test adding VARCHAR that has a default of NULL, an INT with a default and without a default in a Staging server just to see if you can live with any slippage in performance during INSERTs and SELECTs.

This new feature of online DDL is Oracle's answer to Percona's pt-online-schema-change.