Woo Hoo a big kudos to you for even considering fostering a dog!

What you did was the right thing to do. You have to make sure you look at the best interest of the animals, of course your's come first. But I'm sure if your animals won't do good in a situation like that the negative energy would pass down to the foster dog as well, making it hard for him too. She should understand this & should give you a second chance. She should see that you are responsible. If not then I would move on & try it with another organization. Even if it was not your first foster you still have to take into consideration that all animals involved will be as safe & happy as possible. You did the right thing.

I'm not sure what you have already done to prepare yourself for fostering or how deep you want to get into it. But I thought I'd share some suggestions.
Ask a local reputable trainer & behavorist if they can donate some time (or for a small fee since what you are doing is for a good cause which is also all volunteered) to go over some basic care on how to prepare for foster dogs with any type of temperment. Also see if you can call them if a question/problem arrises. Maybe even sit in on a few classes. This will help you prepare for any dog with any type of temperment.
If you are hoping to someday be able to take in any foster dog (even hyper ones like you described) the above sugestions would help tremendously.

A crate is a necessity for just about any foster dog as you do not know their behavior or temperment. Have a few places available throughout your house where you can put the crate. Depending on how the dog reacts to your dogs (or vice-versa) or the temperment of the dog, you may need or want to keep the crate elsewhere. Also keeo a blanket near by. For example if you get a skittish dog it is best to start the dog out in a quiet calm area like a soare room. If you have a dog that barks a lot sometimes covering the crate with a balnket so they can't see ot will quiet them down. If you have a friendly happy go lucky dog that you want around the family but needs to be crated for some reason or another you may want to keep him in the living room or kitchen where all the action & family is.

Toys, especially kongs to keep the dog occupied. At first try to avoid toys that resemble any articles found in the home (old socks, rope toys, they resemble the cheap throw rugs a bit, plastic bottles etc...). Those objects only confuse a dog, they don't know the difference between an old sock or a new one. Also avoid rawhides since a dog has to chew chew chew to make it edible & yummy it only teaches a dog to chew. They think if they chew anything long enough it will become yummy just like the rawhide.

Always make sure the foster dog comes with all his medical history that is available, if you can not properly quarantine a dog that may have an illness you should not accept one.

ALWAYS make sure you can return the dog to the organization should ANYTHING arrise that makes it unsafe or very difficult to keep the dog in your home.

Here's a forum & page with some info that may help, its from a boxer board but it's all about fostering:
http://boxerworld.com/forums/showthread.php?t=18578
http://www.adoptaboxerrescue.com/bri...r_dog_home.htm

I wish you the best of luck & hope that your foster dog is available soon!