May I ask one question? If I create a Segwit wallet on Electrum, can I use that wallet to send/receive bitcoins from Legacy wallets?
Kind of. Electrum jumped straight to using the new Bech32 address format for segwit addresses which only work with wallets that have implemented Bech32 support. There's a trick you can do to get the P2SH nested segwit addresses which are backwards compatible but are still segwit.
You can always send to non-segwit addresses.
Also, will I be able to use the Lightning Network?
No. You will need to use a software that supports the Lightning Network.