sounds like a good approach to me. I'll propose some thoughts, though looking it over now perhaps I skipped too much over your first step, but anyway, here it is. When I refer to plain cost below, it's construction cost, assessed on some kind tightly defined combat-construction metric [ like cost/slots; my preference is cost/(structure*(slots^2)) ] but ignoring other ship aspects, so that "medium cost" has a fairly clear meaning within a hull line and between hull lines, no matter what other differences. I think the use of a good metric for this can do a great deal to guide the balancing job, to the extent a line varies in this metric (and none should vary too much) it should get compensatory balancing of other features, and the rest is mostly adding distinctive flavor in ways that doesn't throw off the basic combat-construction balance. I agree about them all progressing towards more slots; at least some of the secondary characteristics should also get more pronounced I'd think.
high level summary:
Constructed/Initial medium-low benefits (structure, travel speed, construction speed), medium cost
Asteroid: high intra-system stealth with a substantial penalty for a turn after combat, medium-low cost, medium-high structure, low travel speed, medium construction speed
Organic low cost, unlimited fuel, mild structure regen, low structure, low travel speed, slow construction speed
Robotic fast travel speed fast combat speed if/when that matters, medium-fast construction speed, medium cost, medium structure
Energy very fast travel & combat speed, high slots, medium-high cost low structure, slow construction, good stealth
here's some more detail on the Asteroid line thoughts.
I think the main theme of the asteroid ships should be high intra-system-stealth, with secondary characteristics low cost, medium-low structure and low speed. I know there's a bit of that mixed in there already, but I think it should be the main aspect, applying in all systems other than true deep space (i.e., presence of a planet would let it get the bonus; we should probably get it fixed so such systems do show their name like Geoff said they should) and not between systems, and perhaps more of them getting the stealth bonus in systems with asteroids. So every once in a while you could see a litltle flicker of a ship between systems, but if they didn't attack they'd have pretty high stealth. Due to their camouflage type of stealth, being in combat should substantially reduce their stealth fora turn (that might not be exposed to Conditions yet, but last_turn_in_combat is an existing Ship attribute).
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