Seriously, continue to use iPhoto while Photos is still not mature and lacking so many advanced features.
Am I able to start the slide show when I want, or do we all have to sit around and wait for a minute for my screen saver to start up?
You can start the screensaver anytime, if you set up a Hot Corner. in the Mission Control System Preferences. Then just slam the pointer into that corner to start the screensaver. A screensaver has no background music, so you would have to start iTunes with a shuffled playlist.
Or even ACCESS a preselected playlist?
You can access your Music Library, but only search it by album, artist, song.

To continue to use iPhoto update to iPhoto 9.6.1.:
Only iPhoto 9.6.1 is compatible with MacOS X 10.10.3. Usually it will work to reinstall instead of updating:
See this User Tip:
Get iPhoto 9.6.1 if you didn't update before OS... | Apple Support Communities