So what if I can find a P.S of 7-9 Volts and still feed the Mega through the "regulated" input.
The regulator shouldn't heat that much and I won't worry about any backfeeds to the PC ?
That would be better, yes. I want to emphasize that "worry" is not something we do here.
We TEST and VERIFY. WE DO NOT WORRY. This is ELECTRONICS, not ELECTRONICS+PSYCHOLOGY.
Forget about "worry" and put your DMM in current mode and connect it BETWEEN the 5V pin of the arudino with the USB cable connected and the 5V pin of the 5v supply (with all GNDS connected together) and measure the current (on the mA scale) and post a photo of the meter showing what scale it is on and the value shown in the display.
Measure the voltage of the USB 5V and the extednal stadalone 5V P.S. before connecting them together..
Then ask your question again with the TEST results in your post.
We only deal with facts here. If you worry that the earth is going to be hit by a giant asteroid there's nothing we can do about that but if you measure the current between the two 5V supplies and it is very small and you still worry when we tell you there is nothing to worry about then what can we do ?
would it be "better" to use the 7 to 9V supply ? Yes.
Would anything bad happen if you powered the Mega from the 5V pin of the 5V frame supply ?
Let's wait to see your test results.
What Wawa said about the PC being ON when doing the above is correct. If you did use the frame supply connected to 5V pin with USB cable connected you would need to make sure you DID NOT turn OFF the PC. If working with such conditions makes you nervous then maybe the 7V supply is a better solution for you. The USB has a resetable polyfuse that opens if the current exceeds 500 mA but I measured < 1 mA backfeed current when I connected a 5.16 walwart to the 5V pin of an UNO that had a USB 5V voltage of 5.26 V (5.26-5.16= 0.100 V difference).
Do you see the comparison there ? 9 < 1 ma <=> 500 mA)
If you want to learn electronics you need to learn how to verify things "empirically". What that means is that when engineers read datasheets or product info that claims some specification, they design tests to verify the truth of the product manufacturer's claims, and they generate a specification document that contains product specs vs actual specs obtained by testing. They conclude the product claims are either the same as the test results or the product is "underrated" (claims less than it delivers) or the product is "overrated" (claims more than it delivers). At the end of the day, they know what's what. That's what you need to learn. We can tell you anything but you are responsible for what you do, not us. It is your responsibility to verify what we tell you
if you are capable of doing so.
At some point you should reach a point where anyone can tell you anything and you can figure out what's what.
The phrase "Knowledge is power" comes to mind.
"Knowledge about power is also good to have".