Using pt-online-schema-change on Master (MySQL 5.5) - Slave (Amazon Aurora) setup

I want to alter the schema of a table which has around 200 million rows. The change involves adding an existing column to the primary key. The data present in this table is fully compatible with the new schema.

In the current setup, Master is using MySQL 5.5 database. There are 10 slaves which are replicating from the Master. Out of which 5 slaves are on Amazon Aurora which means they are using MySQL 5.6 and the rest are on MySQL 5.5. I am using statement-based-replication to replicate data to the slaves.

Given this setup where the Master is using MySQL 5.5 and slaves are using MySQL 5.6, can I use pt-online-schema-change on Master to make the schema change ? Is this operation safe, considering the different versions ? What are the important things to consider in such a setup ?