86 Chevy 3/4 pu Is voltage regulator causing loss of electrical... lights, horn, dome light, slow electric?

Is the regulator selecting what gets electricity?

No, it's only job is to control the amount of power going to the battery from the alternator. Full power when the battery is low to no power when the battery is fully charged.

If it fails should I replace the whole alternator or just the voltage regulator?

The regulator is inside the alternator. You don't seem to know enough about this system to try opening up your alternator to replace it. Just get a different alternator.

Also, is there any possible connection to this failure and recent replacement of the starter motor?

Only if there is a problem with a common ground to the battery. There is no direct connection between them even though they both connect directly to the battery. If you don't want to try the advise Nomadd gave you, take the truck somewhere and have the entire electrical system checked. I think he's right about bad ground(s).

You should start by getting a $15 multimeter. The only test that counts is how it works in the truck, not on O'Reilly's bench. I'm betting if you measure the voltage between the negative battery post and the chassis when you turn all the lights on, you see something. It sounds like you just have a bad engine to chassis ground.

You might have replaced a good starter motor. That's why you need to spend 5 minutes poking around with the voltmeter before you start taking stuff apart.

What gauge do you have that shows 10A? Ammeters were gone as of the late 70's in these trucks, you mean voltmeter.

As mentioned, buy a voltmeter, learn to use it, and be set for life on simple vehicle electrical issues. They are not hard to learn, and pay for themselves in minutes of use.