Canter work is something that I feel that some gaited trainers don’t do enough of. It used to be that they said that you should never canter a gaited horse. That has since been coming around which is good. Cantering is something that all horses should and can do and it only improves their balance and gaiting.
I would work on getting the canter on the lead line. The more your horse canters the more balance she will get and the better she will canter when we are on her back. Another thing to add to some of your work is teaching your horse vertical flexion. That is when you pick up on two reins softly and hold pressure until your horse softens and drops his nose vertical to the ground. What you need in order to get a good rocking horse canter is collection. Vertical Flexion is not collection but vertical flexion will produce collection over time and softness. Teach your horse how to drop on the vertical and get soft, this will soften his neck, back, and brain.
Also our horses need to learn how to canter in small and big circles. I know it is very easy for gaited horses to canter on a straight line but they need to canter in small and big circles to learn how to balance themselves properly. That is probably why your horse is cross-firing when in the canter, she is not balanced right. Whenever she starts cross-firing slow her down to a gait and get her in vertical and soft again and then start cantering again. She needs to regain her balance and she needs to come out of that canter to do so.
One thing that is going to really help you is a ground pole. When you are in a circle either on the ground or riding, have a ground pole and come around about two strides before that ground pole and ask your horse for a canter. Your horse will more likely pace as you were saying but a horse can not pace while going over a ground pole. So ask for the canter 2 strides away, your horse will come up to the ground pole and will have to change his gait to something else to get over it and if you have speed, you will most likely get a canter. If you have no speed you will most likely get a gait so make sure you ask for that canter and have your horse thinking canter. Now that does not mean that you need to put the gas on fast, just put a little more gas on so your horse knows what you want.
Truly, try it! It will work if you and your horses timing is right!
Thanks and let me know if you have any more questions on what I have said here.