I like the idea of a 18" rear and 19" front, that will allow me to run tires I'm familliar with. A lot of the bikes I've had were set up that way. I'm positive a 19" front will get rid of most if not all ot the light twitchyness.
A larger diameter front wheel/tire will increase trail geometry and improve straight-line stability.
And the larger diameter front wheel improves the gyroscopic stabilization that the wheels provide on a motorcycle
Overall stability should improve.
There probably will be a bit more effort required to get it to turn into a corner.
This is a trade-off, that's probably worth it in this case.
To check your trail measurement, put the bike on level ground, off the centerstand(have somebody hold it upright), take a yardstick or tape measure and use it to extend the center axis of the steering head all the way to the ground, on the same angle the fork legs are on, and mark that spot on the ground. That mark will be forward of the tire contact patch.
Then, drop a plumb line from the center of the axle, and mark that spot on the ground, and it should be in the center of your tire contact patch.
The measurement between those two marks is your trail measurement
Being that the C5 has a leading-axle fork, the axle will be forward of the bottom of the fork leg, so it will vary from the diagram below. No matter, you just drop the line from wherever the axle is located, and with the C5 it will be on the leading axle flange.
I'd be interested to know what the trail measurement of the C5 is, if you want to measure it for us.