Is it possible to keep using iPhoto in parallel with Photos? Is it possible to keep using iPhoto after starting to use Photos?
You can keep using it, but not in parallel.

Any changes made in one app will not be reflected in the other.
Over time, things will diverge more &amp; more.

I would use one &amp; stick to it for now.
When you finally decide to go over to Photos full-time, then do a resync from your current iPhoto state, by deleting the Photos Library &amp; importing again.
Doing that will then erase any work done in Photos &amp; replace it with the latest iPhoto state