For Goblin Grenade can you sacrifice a creature that attacked?
Assuming the Goblins in question survive combat and Goblin Grenade is cast during the Second Main Phase this is perfectly legal.

This is a common strategy for Goblin decks to deal as much damage as they can as fast as they can regardless of the cost to them. While this allows them to make some very powerful plays like sacrificing Goblins to Goblin Grenade, it also makes them very fragile. For example if you are able to counter the Grenade they still have sacrificed the Goblin as part of the cost for paying the spell, so they are down a lot of resources that are going to make it harder for them to win the game. So while in the right deck Goblin Grenade is very strong I would not call it OP or broken.