[Pete Thornton]

loops be called, mains

Main line or mains is good. The side track is a siding.

Few g gauge locos exceed 2 amps, so check the output of your power pack. On a small layout like that you can't really load them up with a heavy 20 car train.

The track may move and the corners take strain from the moving loco. If the track is screwed or clamped together you shouldn't need to screw it down. If you are using regular push-in joiners, I'd suggest some brass or other non-rust screws on the outside of the curves at the corners. Not through the ties, as you have expansion/contraction to worry about.

 

[Pete Thornton]

whats the downside of trying please?

No downside. (Greg just likes people to do things the "right" way.) The engines will just run more slowly and the power pack may get hot and cut out (depending on how it is protected.)

 

[Pete Thornton]

Where would I put the switches in my layout to turn power on and off to inside loop? And I guess I need two power supplies if I want to run the two trains independently?

By switches I assume you mean electrical switches, not track switches that route your trains. The switches usually go between the track feed and the speed controller.
And yes, you will need two speed controllers, though not necessarily two power supplies if the loops are properly isolated and truly independant.