Movement penalties stacking

[Ivalios]

Hello,

While playing a game, me and my companions stumbled about a perplexing combination of effects, and we're just wondering if our interpretation is correct...

The druid of the party cast a Stonecall (difficult terrain which doubles the cost of movement) and then cast Sirocco (causes fatigue and exhaustion later on)

So, right now, just to be sure: Fatigue/Exhaustion decreases movement speed by half (From 30ft to 15ft for your standard character) and to move 3 5ft squares, it'll cost him his full round... if the math if correct...

Now come the interesting part: Our Arcanist cast Mythic Slow on the poor chaps, causing the staggered condition and decreasing movement speed down to a quarter. Are we only going to be taking the biggest penalty here (1/4 movement instead of full) or are we stacking them?

Because, to us, it looks like this 3 spell combination will lock down a group of enemies, as they cannot full round (Staggered)thus negating the minimum movement rule (See below)

Minimum Movement Rule:

Minimum Movement: Despite whatever penalties to movement you might have, you can take a full-round action to move 5 feet (1 square) in any direction, even diagonally. This rule doesn't allow you to move through impassable terrain or to move when all movement is prohibited. Such movement provokes attacks of opportunity as normal (despite the distance covered, this move isn't a 5-foot step).