is it safe to use a water (from big aquarium with many fishes) to new setting medium bowl tank with 2 fish?

Answer 1

Along with adding some of the water, I'd also suggest moving a handful of gravel and your filter pad from the old tank.

Using the gravel and filter will add the beneficial bacteria that break down your fishes' wastes - otherwise, the ammonia and nitrite will build up in the tank, and both can be toxic to your fish. The bacteria attaches to surfaces, so you won't get much if you only transfer the water.

And I'd mix about 1/2 old water and 1/2 new - this will reduce any ammonia, nitrite, and nitrate from the old water.

Answer 2

Sure As Long As The Water Is Cycled And The Quality Is Good, There Should Be No Reason Why Not.

Answer 3

Not only is it safe, it's beneficial.
(as long as it's clean and the big tank has proper filtration and chemical balances)

The "old" water is chemically balanced for fish, it contains beneficial bacteria, which will start the new tank cycling, and should be close to the right temperature right from the start.

Good luck.