I'm not sure if this question is floating around anywhere but I'm not entirely sure. Are you able to TK a horde stack of say, 5 as if it was under normal single base/normal size conditions, or is it because its a stack that you cannot?? Thanks.

There is no rule preventing the TKing of stacks.

Yes, TK is a great option to move Horde Stacks since they can't be carried and many of them have low speed values.

On a similar point (as I misread the question and it got me thinking)... can you TK a Stack into another Stack? Like if I have 2 Stacks that each have 2 in them (with an M of 7), could I TK one of the Stacks "into" the other Stack for them to combine? I don't think the rules say it has to be from a move action specifically, just that the action ends, does it?

Yep!

"Unlike other characters, identical horde tokens (those with the same set symbol and collector number) can share a single square on the battlefield by being stacked,"
"Two or more identical friendly horde tokens in the same square after actions resolve is a stack"

Only unstacking requires a specific action (a move action to unstack), forming a stack just requires them to have actions resolve in the same square.

Huh... I never considered that before and may have just been stuck on the "unstacking" requiring a move action. Wow... I need to try that now. LOL!