Both are correct, the first one à tout à l’heure (without a s at the end) is the most common.

And the second one à toute, alors, is more familiar. It is a contraction of the first example (https://fr.wiktionary.org/wiki/%C3%A0_toute), with alors added at the end, it can be translated to :

See you later, then.

But they mean the same thing in the end.

Are you sure it was “À tout alors”? I think you’ve heard “À toute, alors !”. Here is an exemple of context:

— Je vais à la conférence du logiciel libre cet après-midi.

— Ah oui ? J’y vais aussi !

— Cool ! À toute, alors ! / Cool ! Alors à toute !

A translation might be:

— I’m going to the free software conference this afternoon.

— Oh really? I’ll be there too.

— Nice! See you later, then!

And the first one, À tout à l’heure is right. This is the full form of the expression. Even if you can say À toute, alors, it is more colloquial indeed.

You misheard – à tout alors doesn’t exist, that’s your interpretation of à tout à l’heure. The most common really colloquial expression is à plus, short for à plus tard.

Either one can be followed by alors, as any other sentence would in conversation – just like Americans would start anything with so