are individual satoshis as secure as an overall bitcoin?
When people "hold Bitcoin", they actually have access to a private key that allows them to sign for one or more unspent transaction outputs (UTXO) of the corresponding Bitcoin sum. The amounts in transaction outputs are in fact specified as a count of satoshis. 

All unspent transaction outputs are tracked by each full node on the network, therefore, it is no more likely that a smaller amount is going to be forgotten rather than a greater amount. However, since transaction fees are paid for transaction size and not transacted amount, smaller amounts can have a much higher relative cost to be spent than larger sums.