Do you always need at least 1 Egld to restake or can you do it with smaller amounts?

You can restake with small amounts - i.e. less than 1 eGLD. The 1 eGLD minimum is just for the initial stake when you joined a staking provider. Just be aware that there are some very small fees involved when staking your rewards (as there is for all txns) so I recommend using @TrustStakingBot to determine the best interval to stake your rewards.

You can restake smaller than 1