That is one problem with Homeworld in general, not as many ships and ship roles as Sins, which makes them difficult to add to Sins.
Well, I would recommend the following:
Strike Craft
Interceptor and Assault Craft = Fighter
Bomber = Bomber
Gunship and Missile Corvette = Corvette
Pulsar Gunship and Laser Corvette = Heavy Corvette
Frigates
Scout = Scout Frigate (for lack of a better choice)
Torpedo Frigate and Plasma Lance Fighter = Light Frigate
Ion Cannon Frigate and Heavy Missile Frigate = Long Range Frigate
Flak Frigate and Assault Frigate = Flak Frigate
Variant of Light Frigate or Long Range Frigate = Siege Frigate
Marine Frigate and Infiltrator Frigate = Colony Frigate
Cruisers, questionable at best.
Support Frigate = Light Carrier (support was largely cut after Homeworld, but the Support Frigate repaired ships and refueled Strike Craft)
Defense Field Frigate = Defensive Support Cruiser
Command Corvette = Offensive Support Cruiser
Destroyer = Heavy Cruiser
Minelayer Corvette = Minelayer Cruiser
Capital Ships
Battlecruiser = Battleship
Carrier = Carrier
Crew Transport (Bishop Transport) = Mothership (Colony)
Progenitor Keeper = Support
Progenitor Dreadnaught = Siege
Mothership = Starbase or Titan
? = Trade Ship
Resource Collector = Refinery Ship
Shipyard = Frigate
Tanis Derelict = Capital Ship Factory
Mobile Refinery = Orbital Refinery
The Strike Craft and frigate roster should be pretty much set, its the cruiser and capital ship roster that is a problem beyond the obvious equivalents.