New Tank Question

I would run the new tank for a day or so just to make sure the tank doesn't leak and the electric bits work properly.
Then if you want to transfer everything over, just move it all at the same time. As long as the filter decor etc move at the same time as the fish there shouldn't be much problem, if any at all. You will lose the bacteria on the glass itself and gravel (as it will be a good idea to wash it) but the vast majority of the bacteria will go with the fish. Just monitor your ammonia and nitrite for a few days to make sure. You won't be able to get all the water from the old tank as you'll be surprised how much muck is in the gravel. The easiest way is to put everything (fish included) into a bucket or two, making sure the fish one is covered as fish can jump. Then remove some of the testing water from the new tank, take out the water from the old leaving a couple of inches in the bottom saving as much as you have room for, scoop out the gravel and wash it. The starting with the gravel, put everything into the new tank and top up with new water if necessary. This is what I did in January when i moved everything from a 125 litre to a 180 litre.

If you want to run both tanks, just move old media into the new filter in the same ratio as the amount of fish that will be in each tank.

I know the feeling - my husband complains about my three tanks