1. Impedance matching is how the transformer supply side (primary) sees the load (connected on the secondary), its calculation is important only when u intend to follow max. power transfer theorem.
2. Frequency change may effect the impedance of the transformer resulting in reactive losses, other than that, no significant decrease in efficiency occirs.
3. The current rating is determined by both the thickness & the number of turns.
4. Remember that current is simply the flow of free electrons. Giving only 2 turns on primary & 1 on secondary would result in
a. Very low power rating (current handling capability would be very low)
b. Very low impedance which could result in shorting the input power supply if it is working on low frequency. Corollary to it, the greater the frequency, the smaller the transformer gets giving the same power ratings.
A two turn primary & one turn secondary transformer, if made & daringly connected to the mains power will, until it blows up or some circuit breaker trips, will not have as much power (VA) rating as a 2000/1000 one.