Yes you can do that. You can manually change a monster's position if all of the following are true:
- The monster has not be summoned (including flip summon and special summon) or set this turn.
- The monster's battle position has not been changed manually or changed by a card's effect (it's own effect or other card effect such as Book of Moon).
- The monster has not attacked yet.
In fact, that is the correct way of playing Stealth Bird, Swarm of Locust, Swarm of Scarabs, and Guardian Sphinx. You set them on this turn (you can also summon them in ATK position, but you loose the surprise), then on your next turn, flip summon them, attack if you want, then use their effect to turn them face-down deffense position.
Now, the other way won't work. If they were in ATK position, you can use their effect to flip them face-down, but you can't flip summon them at this turn anymore.
Also, you are allowed to flip summon as many monster as you wish.