As long as you don't need all these voltages, and it can operate on 12V only then you will have no trouble drawing that small amount of amperage from pin 5 of the serial port. One thing about this though, expect to make your own serial cable that just has pins 2,3 and 5 available. The easiest way to do it is to buy one and cut it and use a multimeter to do a continuity test until you find which colored wires go to which pins. But try it before you strip a perfectly good serial cable of course.
I have no idea about the anteanna, I'm not radio expert, but I expect the answer is no if your GPS unit needs an active antenna that can handle a specific voltage. For instance, I needed a 5V active antenna for my Oncore unit to work well.