Generally cruisers already stand up to a slugfest very well. A cruiser can soak up a lot of punishment. Frigates could use a bit of love though, at least in regards to them being so squishy. Frigate armor is fine, its just that frigate shields are more of a suggestion of shields than providing actual protecting.
Leave frigate shields thin, but allowing frigates to recharge ships around them, including cruisers or other frigates, would give Empire frigates a lot more staying power. They still have a weakness due to the huge shield bubbles, but at least the shield bubbles would remain up longer.
Frigate defenses are so weak that its often times not even worth giving them any defenses at all. Pack them with lots of guns and engines, send them in, and hope they kill enough things before they explode. Allowing frigates to recharge each others shields would give them a significant boost to viability.
I do love the idea of modules that allow ships to support each other. Shield recharge modules, repair modules, maybe even targeting modules, or some way to intercept incoming fire even if its heading to another ship. Large shield bubbles maybe? A module that provides a gigantic shield bubble which can protect multiple ships even if the ship that has the module is not the one being shot at?