1) OK. But Segwit with Electrum and Trezor should be compatible with other Segwit Wallets?
Trezor is using "3"-type (aka P2SH-P2WKH) SegWit addresses... I'm not sure if you can actually specify if Electrum should import the SegWit or Legacy wallet from a hardware wallet like Trezor or Ledger... I think it will just import the "legacy" wallet "xpub" from the device.
2) What happens when I try to send FROM "other wallets ("3" type)" to Electrum SegWit ("bc1" type).
Is there an error? Or would I lose my coins? What would happen?
If they support bech32 addresses... it should send just fine... if they don't they will more than likely give a "invalid address" error.
3) Is there a reason never to use Segwit and stick with the legend addresses? Can this be any advantage?
Any fork that doesn't support SegWit will make claiming fork coins difficult. Also, there is currently no method for signing messages from a SegWit address ("3" or "bc1")... so participating in some ICO's and/or AirDrops could be problematic if you have a SegWit wallet.
Also, it will mean you can only migrate to a SegWit supporting wallet in the future...