I want some posts to be full wide (galleries). And others only with sidebar. Is this possible?
You can set page templates by selecting one that comes with simplified (pro) theme, however there is no similar wordpress mechanism for post templates.
You can however do that either by creating a child theme which includes condition for galleries (tag, category or post type).
Or, which is a simpler way, use plugin that allows creation of post templates. This should do fine. Refer to the documentation of that plugin to customize everything the way you like it.