My read on that was that the speeders couldn't take them down. I thought perhaps an X-wing or Y-wing could do it, but they needed the X-wings to escort the transports. Never saw a Y-wing on Hoth.
The "armor is too strong for blasters", to me, was always another one of those "true from a certain POV" things... They had finite time to bring them down before they reached the shield generator, took it down, and then essentially all hell would break loose. So I've looked at it, and EU always sorta supported this, that blasters CAN take out the AT-AT's, if you hit them repeatedly at points, have a stronger blaster/weapon, or hit it at weaker spots that are difficult to hit in general... Like I say, EU always supported that, and it makes sense, but the AT-AT's weren't slow (Someone measured their speed on film once, and they're shockingly fast, and nimble too). The one that stomps Luke's speeder is on him like stink on poop pretty quickly, and was a good distance away at first.
So basically once the AT-AT's/etc. stomp on through the shield (if shielding technology works like that, in that repulsors cannot pass shields, but items can "roll" through or walk through them), it was a race to see how much crap you can get off the planet... That jives with Luke's 2-1B conversation too, in that he said not to worry about this or that, they just needed to get the main stuff off planet.
Also, the planet was JUST set up... Did they have time to get all their main weapons emplacements up and running? Would they have had a better defensive set-up given more time? Who knows really, all conjecture, but again EU stuff kind of implied a siege of Yavin took some time whereas a siege of Hoth was much quicker.
Regarding starfighters, I think they were more worried about the transports/personnel escaping, so starfighters (which seemed to be stretched thin), had a different duty. I wouldn't think cold would impact them... I mean, Hoth's cold sure, but is Hoth colder than outer space? I can't buy into that then, that a fighter can't take the cold. Hell, it gets so cold in upper atmospheres you have to have special suits to survive, but on Hoth they're just wearing their Columbia gear. Luke's XWing on Dagobah I always just chalked that up to Dagobah kind of being a weirdly mystical force-y kind of place, and never thought much of it being humidity or whatnot. Not to say that's wrong or anything, I just always figured it was part of Yoda's way of staying hidden and the planet having the cave, and all those unanswered questions surrounding Dagobah. Yavin's a rain forest afterall, pretty humid too and stuff. But that's just my point of view on it.
But back to Hoth, if fighters are stretched thin, and your goal is survival of the personnel and main equipment, your fighters will get the job of protection (2 per transport, and less as things got worse). The Speeders just had to slow down the walkers and hope they buy enough time, which they seem to. I'm guessing the fighters being sent with the transports is because, again, time is limited, they know they can't repel the attack with what they have (regardless if they have fighters come in and take out AT-AT's more efficiently... more will be on the way), and eventually they'll just get encircled and entrapped. It's a no-win situation where the only positive is escape of as many as possible.
I've always also felt that, with the shield not being planet-wide, but rather just a relatively small surface area of Hoth that encompasses the base itself, the Speeders, with their tighter maneuverability and such, would operate better inside the shield itself, whereas X-Wings and Y-Wings, etc., don't turn as tight (that was always the Speeder's "advantage" in EU stuff... They're unshielded and not terribly well armed, but were tough despite this, and were highly maneuverable, more so than TIE's even which generally outperformed Rebel fighters overall).
I also seem to recall something that the Rebel fighter maneuverability is also "hampered" a bit in atmosphere compared to space, though it seemed like it wasn't a huge difference, but a reason they invested in airspeeders (plus they're less expensive). However, Imperial fighters tended to perform better in atmosphere, and thus they didn't put much stock in airspeeders as a weapon they'd need as much.
I totally see the POV though, that the Rebels never encountered AT-AT's, by the dialogue... This is like everything else in the films where you can take a very different POV of how Star Wars is/works if you ONLY look at it from the film POV. In that sense, the Rebels are a tiny force, EVERYTHING they have is at Endor, they rarely willingly engage the Empire, and so on. It's a really different way of looking at things and just as valid a POV on it.
Another question... Are the Rebels working in "Cells" by the point of ESB or are they much more of a cohesive military? I think, even during the SW Rebels era, they're a cohesive military core, but still with cells coming into the fold eventually. Thus they seem to have a military structure overall, but then you have other militias, etc. I like to think of the Alliance as the US military in the Revolution... There's the continental army, or the "core", and then you have the militia, guerilla forces, and other groups, who share the same cause, but aren't absorbed into the core... Or for a different era, the NVA, and the "Viet Cong" which was comprised of a lot of other forces, during the Vietnam War.
Another interesting thing to think about, regarding the AT-AT's and their seemingly strong armor... If they're impervious to the blaster fire, why not make EVERYTHING out of that? Or is it that they're just impervious to weaker blaster fire? Or that they can be hit in spots, and taken down, but it takes longer than the Alliance had at Hoth, given their defensive situation and lack of prep? If AT-AT's were so strong they couldn't be taken out by blasters, at all, why not make EVERYTHING out of that kind of armor? Or are they just really tough and it takes the right shot, in the right situation, or having the exactly right weapon(s)?
Regarding the AT-ACT/AT-AT... Like I say, I think they're just different models for different reasons... Like a Jagdtiger to a Tiger to a Sturmtiger... Same chassis, all 3 different uses, strengths, and weaknesses, to put a real world spin on it.