How were the first coins spendable?
If the first coins were not spendable, how was Satoshi be able to transfer the first 10 BTC for pizzas?


It wasn't Satoshi (it was Laszlo Hanyecz, 1.5 years after the genesis block), it was 10000 BTC for 2 pizzas, and they were simply not the first coins.


  Where did these first 10 BTC come from, if the first 50 BTC reward were not spendable?


They were mined. Simply not in the first block.


  And how could then there be more coins, if you first need some transactions in order to fill the blocksize and mine the next block?


There is no such requirement. Blocks do not have to contain any transactions (apart from the coinbase transaction, which creates the new coins).