But how would I create a segwit address that starts with 3? Does Electrum support that?
Electrum supports it, but can't create them. It's confusing, and there isn't much information about it, which is one of the reasons I'm still not using SegWit by myself.
According to
jreuab @ Reddit, you'll need to "manually import a BIP39 seed with a BIP49 derivation path."
Also as you said that the transactions will be slightly bigger than the conventional one, will there be any issue in confirmation?
You'll need to pay a slightly higher fee again, or wait longer.