Skipping Android Security Updates? I am rooted on my Nexus 6P and I used Flashfire to install the March security update. I didn't install any of the previous updates and haven't installed any after the March one. 

This has made me think of a few questions:

Does the March update contain all fixes from prior updates as well?

Do I have to install each incremental security update or can I just install the May one without installing April's?

Should I go back and install the previous patches and if yes, will this negatively affect anything?

I have noticed a lot of random reboots and the device running hotter than before, although I am not sure if this is because of the manual updates or a bad app.

Any insight is appreciated.
You do need to install security updates incrementally, as each release has its own /system image that has had all the previous patches already applied to it. When installing an update, most of the time, /system is completely wiped and then replaced with the new image in your update file.

About the negative performance, a common issue is that the update is dirty flashed, meaning that the /data partition was not wiped with the new system image install. Try first wiping your Dalvik/cache partitions from your recovery. If that doesn't help, you can try wiping /data to see if that will solve your performance issues.

Wiping /data will remove all user data from the device, so be sure to back up any important data before performing a factory reset.