Well, you probably should try and find a PDF of the 5th Edition rulebook somewhere. A good portion of it is real basic n00b stuff, which could be helpful for you, however a lot of it is really, really nice since it contains all sorts of info about rules and weapon types. Note that that's just the rulebook, it contains no unit lists for any race, so you'll need to grab the codices separately.
So for your first question, yes, other races can take vehicle squadrons. Tau Piranha and Tetra speeders are in squadrons, as are Space Marine Land Speeders. To the best of my knowledge though the Imperial Guard are the only ones who can take main battle tanks and self-propelled artillery in squadrons.
For question number two, any army can have a maximum of two HQ choices, six troop choices, three fast attack choices, three elite choices, and three heavy support choices. No army however can have anything less than one HQ choice and two troop choices. And to represent the without number horde that is the Imperial Guard, an IG army can get for a single troop slot a Platoon Command Squad, 2-5 Guardsmen squads, up to 5 Heavy Weapons Squads (each with three heavy weapons), up to 5 Special Weapons Squads, and a Conscripts squad for use as a meatshield, which they do wonderfully since they are 4 points per Conscript and a single squad can have up to 50 of them.
Of course, the most devastating weapon in the IG arsenal is not its tanks, nor its infantry, but a man known as Lord Castellan Ursarkar E. Creed who can be taken as a HQ choice and allows any unit in a player's army to have the Scouts special rule, which allows them to Outflank if kept in reserve (which means they get to move in from one of the table edges when deploying from reserves). So yes, as some of the less outrageous Creed writefaggotry says, Creed can indeed make Leman Russ tanks appear out of nowhere, since his rule does not include any limits on what unit can be given the Scouts rule.
