Yes, creature tokens, including populated copies, enter the battlefield, so any cards that care about creatures/creature tokens entering the field would trigger with populate
You can continue tapping creatures to station it afterward, which wont boost the Sweeper itself, but it helps fuel several cards in the set that care about tapped creatures
and Bruna, the Fading Light and Gisela, the Broken Blade meld to form Brisela, Voice of [autocard]Nightmares[/autocard]
While this card has certainly seen more play in Legacy, the ability to jam the Monarch into play and scale with a leverage that Forth provides is also quite good in a format like Vintage where there aren't always going to be creatures to steal back the Monarch
The first of them during his turn activated the ultimate ability of Sorin Markov, targeting you