They both rebalance so the main campaign is impossible. They both add nothing to playing the single player campaign. What more do you need to know?
I got a Rheinland Valkyrie once (that IS the heavy, right?) after the main campaign was over. I was impressed with its handling characteristics and durablity, and Rhineland's best guns are fully a match for any of the experimentals while still being a teir or two lower. However with only four guns and an aft-only turret, I didn't feel the Valkyrie had the firepower requisite to a heavy fighter.
Yes. The Valkyrie is the Heavy Fighter.
Having four guns as compared to five or more has an advantage in itself. You can fire longer.
The other nice thing about vanilla Freelancer is that they give a small number of ships, so after a lot of flying here and there, you'll more or less know what each ship is called and classified. I'm going to give it a go now.
Starflier - Civilian Light Fighter
Startracker - Civilian Light Fighter
Hawk - Civilian Light Fighter
Falcon - Civilian Heavy Fighter
Eagle - Civilian Very Heavy Fighter
Piranha - Bounty Hunter Light Fighter
Barracuda - Bounty Hunter Heavy Fighter
Hammerhead - Bounty Hunter Very Heavy Fighter
Patriot - Liberty Light Fighter
Rhino - Liberty Freighter
Defender - Liberty Heavy Fighter
Bloodhound - Pirate Light Fighter
Mule - Pirate Freighter
Wolfhound - Pirate Heavy Fighter
Cavalier - Bretonia Light Fighter
Clydesdale - Bretonia Freighter
Crusader - Bretonia Heavy Fighter
Drake - Kusari Light Fighter
??? - Kusari Freighter
Dragon - Kusari Heavy Fighter
Anubis - The Order Heavy Fighter
Banshee - Rheinland Light Fighter
Humpback - Rhienland Freighter
Valkyrie - Rhienland Heavy Fighter
Dagger - Border Worlds Light Fighter
Dromedary - Border Worlds Freighter
Stiletto - Border Worlds Heavy Fighter
Sabre - Border Worlds Very Heavy Fighter
Legionnaire - Corsair Light Fighter
Centurion - Corsair Heavy Fighter
Titan - Corsair Very Heavy Fighter
I'm not too certain about the Civilian ships, nor do I know what the Kusari freighter is called, but there you go.
Has anyone tried the Humpback? I did once, and I regretted it greatly.