Were casualties heavy when the confederates bombarded fort Sumter?

No, not at all. In fact no one was killed, or even seriously injured. After more than a day the Rebel bombardment started a fire, which Union troops feared might eventually threaten to reach the Fort's gunpowder magazine, which would have exploded if the fire did reach it. That would probably have done a lot of damage and probably killed some defenders. This fire was the reason given for the Yankees accepting terms of surrender. The terms specified that the defenders of the Fort would not be held as prisoners but would be sent back to the north, and this was done. Also as part of the surrender agreement the Yankees wanted to hold a ceremony to lower the flag, and to fire a cannon salute to the flag while this was being done. This they were allowed, and when they fired the cannon salute their cannon exploded, burst, and this mortally wounded a man serving in the cannon crew. This was the only person killed in the entire Fort Sumter affair.