It's usually better just to salvage the enemy ships and their technology then to wait until you develop it. Salvage whenever possible. The cost for 12 salvage corvettes is about 2400 RUs. But with those corvettes, you can salvage 8000-10000 RUs worth of enemy ships. Multigun corvettes are good against other corvettes and fighters.
Heavy cruisers sound good, but they're way too slow to keep up with an attacking fleet.
Pretty useless unless you get into a huge firefight or are playing defense. Carriers are a must for any fleet. They can take lots of damage and make great distractions for the enemy to attack. Assault frigates are good against other assault frigates and corvettes. Missle destroyers are great against smaller ships like corvettes and fighters.