It is possible to state there are probably legitimate rationales behind all the items you've stated.
AT-ATs would be sensible devices if the ground was swampy or did not support tracked/wheeled vehicles in some way, and height can be an advantage.
As for the weird designs, remember the Empire was until recently a Federation (Republic is really the wrong word) and each world probably had its own vehicle and arms manufacturers. The Empire probably has a few of the hideously expensive Star Destroyers and has more or less standardised equipping its basic infantryman (Stormtrooper), but hasn't got round to such standardisation for it's mid range vehicles. Also just because its an Empire doesn't mean that politics doesn't exist, and every world would be pushing the products of its own manufacturers.....
It's commonly stated that you fight a war with the equipment you have, not the equipment you wished you had....