Paste or liquid wax usually contains petrolium distilates to keep the wax soft. If you have ever frozen liquid wax in the winter by storing it outside it is easy to see this as they seperate. Those chemicals are the problem with getting it on plastic and rubber. They leave a "haze" on the surface that you cannot get rid of.
Some liquid waxes don't contain these chemicals, but then they don't contain alot of wax in them either. If you use a solid wax, then it generally is very hard, but contains no distilates, but is harder to spread around, and remove. It also leaves the "hardest" longest lasting finish.
I use Meguirs gold class because waxing is a pain in the ass as it is, and I would rather do it a couple more times a year than once with solid carnuba.
As far as mixing waxes, I wouldn't worry about it. If you are going to worry about it, just wash the car with liquid dish soap and it will remove the wax and you can start over. Or you could spend more money on a car wash that is made for stripping wax. Either way...
cheeRS,
Greg