No, you cannot join copper and galvy directly. A brass fitting needs to be between the two to prevent electrolysis. You could also use a special type of union that has a gasket between the two halves. It is called a Dielectric union. If this junction is between pipes on your water heater and the cold water supply line, you could use a fitting made for that that also is a back flow preventer (ie a check valve).
No. Compression fittings are NOT for steam pipes
Since most water is carried to houses through copper pipes, yeah.
Copper pipes are perfectly safe. If you were to consume too much copper you could then suffer from copper toxicity, however metallic copper is not very soluble in water, and you will not get any excessive amount being dissolved by using copper pipes. Also note that in very small amounts, copper is actually beneficial, since it is a necessary trace element. So copper pipes are good. Or what I was told is that the Romans drank out of copper pipes and they all went mad but that's what i heard.
Definitely not! Galvanized when heated will emit toxic fumes.
because the pipes don't rust as easy, and silver and gold are too expensive. That is another persons answer. Here are the main 3 reasons plumbers use copper pipes. Copper pipes are relatively non-toxic-unlike lead. Copper pipes do not suffer corrosion-as iron does. Copper pipes are relatively soft and easy to work with-unlike both iron and lead. However more and more homes are now using the plastic pipe such as Pvc and Cpvc which seems to serve the same purpose and are also safe, although both can be used, you still need to find out what code calls for in your city or town an abide by it.
Copper (as a metal or alloys) is safe.
50psi is a safe route. No problems there. the fittings can largely effect the pressure effect noticed once the water leaves the pipes.
It is safe, but it will rot out. It is used to prevent electrolysis, but that is pretty much it nowadays. Pex is more commonly used for water pipes.
depending on what area of the country you are in...16" should be safe enough. you should be careful of rock coming in contact with the piping as it could result in abrasive punctures over time.
It is safe to cook in copper cookware. However it is not advised to cook baby food it copper cookware because it harms the effects of certain vitamins. In addition very acidic foods should not be cooked in copper cookware as they may contain toxic amounts of copper.