Segwit transaction from mixed addresses (segwit and non-segwit) Is it possible to have partly segwit transactions? I.e. some utxos from segwit addresses and some from non-segwit addresses.

Asking for the situation of Bitcoin core v0.15 wallet with coins in segwit and non-segwit addresses. If you send a transaction will it use one or both address types for the transaction?
The right way to look at it is that every transaction input has a witness. For legacy outputs being spent, that witness is just empty.

So yes, mixed SegWit and legacy inputs are possible. SegWit transaction serialization will be used, and the witnesses for all legacy inputs will be empty.