Roberto Di Meglio
2) Yes (see below)
3) No, No.
You handle each ship through:
1) a ship log (stats/damage track), which is included with each ship
2) a ship mat, which includes a "frame" to the ship log, with all kinds of neat boxes and squares to keep things in order. Each ship mat also includes the set of counters necessary to handle a ship.
1) The Starter Set includes 4 Ship mats (and counters). So, as long as you only play with 4 ships at one time, you don't need additional mats.
2) When playing with basic or standard rules, it's very easy to keep track of what a ship is doing even without the ship mat and counters (essentially, the ship is only firing or reloading). So the mat and the counters are nice to have, but not a strict requirement.
So if you play 5+ ships with these rules sets, you may handle them without the mats without many problems.
But when you play 5+ ships with advanced rules, you need to plan and track crew actions, and the ship mats and counters become really necessary.
3) The mats in the Starter and the mats in the "Additional mats" (as the name suggests) are the same thing. Ship logs are included with each ship. The Starter Set includes 4 Ship Logs for the ships included in the set. Each Ship Pack also includes the Ship Log of the ship it contains.