I've been doing this for a while (lucky bastard as I am), so in 4th ed I have a few general rules I follow.
1. Lower force spirits (1-3) I consider to be the grunts of the spirit world, and my experience GMing shows me that they are not game breaking at all, and aside from the pet factor (taking time from other players while the mage manages their pets), I don't see them as a problem.
2. Higher force spirits (4-6) I treat as much more individually motivated, and unless the player does some serious role play or actions to appease the spirit (or intimidate it), I almost always use the spirit's edge when resisting the summoning / binding. I dropped 20 boxes of Stun Drain on a player that was trying to bind a rating 5 spirit with exploding sixes.
3. Be fair! I use the same rules for my NPC's as I use for the players. I KO'd a major NPC of mine when she tried to to summon a Force 8 Guardian spirit.
I am on the side that mages should be powerful, especially for how many BP (or Karma if you will) it takes to build one. But, my Shadowrun world has always had the maxim "Geek the Mage!".
As far as ghouls go, being Dual Natured will not bypass the Spirits's Immunity to Normal Weapon's and I'll give you a very good example why.
A Phys ad with Astral Sight merely makes them dual natured (and will allow them to take Astral Combat), but they still need Killing Hands to bypass the immunity to normal weapons.
Ghouls can not attempt astral combat unless they have the skill (no default for it)
What Ghouls would do is hide. And if they can't get away from the rampaging spirit they would swarm it while others would flee. Not likely a Ghoul even with 4 buddies could get a decent enough hit (they would need 9 net hits to affect it), a swarm of ghouls using edge have a chance in theory.
Honestly, if you have three plus players anything less than 5 ghouls are not going to pose a real threat anyway unless they get the drop on them. What is nasty is that unless the Mage has Masking, and some ghouls get past the spirit, they can assense who the "real" threat is. I've seen what a pack of ghouls do to a mage if they get too close, and it isn't nice.
Last rule, you are the GM, if you were planning on using 6 ghouls, and your mage smokes them in one round leaving the rest of the players to twiddle their thumbs, add more! One of my players' favorite moments in one of my games:
Player Chris, playing a combat mage using Stun Ball, "How many gangers are there?"
Me,"You gonna stop and count them?"
Chris, "Can I just guestimate?"
Me,"Sure"
Chris," OK how many?"
Me, "Enough."
My players cracked up. It meant that I would provide enough fodder for all of them to get involved and provide a challenge. I wasn't going to let the mage cast one spell and wipe all of them out (Especially back in 3rd Ed, talk about pwnage!)