Zip ties should be alright but an O-ring would be your best bet. I Have a cheap o-ring kit from ebay
http://cgi.ebay.com/GASKET-407-PC-RU...d=p3286.c0.m14 that works very well.
I used a properly sized o-ring to fit around the axle pin to keep it from popping out. The little cir-clip things included with the adapters DO work but they are a serious pain to install. I Find that the o-ring supplies sufficient pressure to keep the pin in and are easy to pop on/off.
The washer should be fine. Just make sure its not TOO tight or you will have driveline drag which will heat up your electronics really quick. Maybe even run to your local hardware store to see if they have what you need.
I too really like the extra ground clearance and the width of the 1/8 buggy tires is about perfect (good traction but easy enough to slide around). Plus the super low profile of 1/8 buggy tires is a tremendous increase in handling. No more sidewall flex!