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robot_localization, like many ROS packages, contains multiple nodes, but you certainly don't need to use them all. As of this writing, it contains an EKF implementation (ekf_localization_node), a UKF implementation (ukf_localization_node), and a node that aids users in working with GPS data (navsat_transform_node). To answer Q1, yes, you can just use ekf_localization_node.

For Q2, @Anas Alhashimi posted a good link. What you really need to know is (a) the data and units required for the Odometry and Imu messages, and (b) a way of converting your raw data into the format required by those messages. For calculating odometry in (a), this looks like a good article. My guess is that your IMU will give you information about the data it produces; it may be a straightforward process of just filling out the data fields of the message and publishing it. Be careful, though. robot_localization makes some assumptions about your IMU data. See this post for more information.