Deleting space on my iphone/icloud If I am storing photos on my Dropbox or Google Photos, can I delete them from my iCloud and still access them through these other apps?
Yes, those services are not connected.  

However, once you delete items from iCloud, they'll be deleted from every device you have that uses that same Apple ID.  iCloud is a sync services whereas Dropbox and Google Drive are cloud storage that can also be used for backup.