The Red Head, Lyta, indeed was featured in The Gathering, but, The Studio wanted someone sexier, so, the part of Talia was written, and Lyta was removed from the canvas afer The Gathering
It isn't that Pat was insufficiently sexy, it's that she had other obligations. Remember, most pilots fail, and B5 wasn't picked up until more than a year after the Gathering was filmed. She didn't have a contractual guarantee so she took other work and wasn't able to come back for the first season or most of the second. Incidentally, she just gave birth six weeks before this episode was filmed.
JMS originally intended Takashima to be the mole. When her actress didn't come back for the series, JMS has to switch it to Tallia. It was originally intended that Tallia have an opening to come back, the recording of her emotions. But Andrea Thompson didn't want to return and Pat Tallman was available again, so it worked out the way it did.