First of all, you cant select soldiers or any other type of portal minions. Maybe you were imagining things, or maybe simply you confused soldiers with minotaurs.
Second, I dont think there is a keyboard shortcut for "select soldiers", and if there is, maybe it is left from when the game was still in Beta version, where there were other modes that were removed very quickly by the developers (change of mind, bugs, or some other reason).
3rd, you can select all your minions (only if you have a general) with a shorcut key, just your siege idols, or just you monks etc.
There are different shortcuts but only for your own minions.
Creeps or Portal Minions as you call them, act on their own, following always the same path along the map.
You cant control them.