Can I activate the effect of Swarm of Scarabs after being SpecialSumoned in face down with insect imitation ??

Question

and destroy a monster NOTE: INSECT IMITATION summons him in face down defese position !!!

Answer 1

No you cannot.  Once a monster is Special Summoned, you can't change its battle position again until your next turn.  A good example of this is Treeborn Frog.  If you choose to Special Summon Treeborn Frog during your Standby Phase you must choose which position you want to Special Summon him in.  Once he is Special Summoned he must stay in the chosen position until your next turn unless you change it with a card effect.  

Another thing I should note is that should you Special Summon Swarm of Scarabs in face-up position, then you can change it's position by activating its own effect.