Seeding the cycle

You can indeed swap filters around to get some bacteria growing, but all the bacteria in all your tanks will only equal the amount of ammonia in those tanks. How many fish you have is related to how much bacteria you have. To get more fish, you need to grow more bacteria, not just share bacteria among the tanks.

Here is what I would do:
1) Put the right sized filter on each tank.
2) Put most of the cycled media in the filter that is serving the tank where the fish live. Keep a good colony going so the fish do not suffer from the ammonia or nitrite.
3) Put a little bit of cycled media in the new set up (whichever tanks do not have fish).
4) Do the fishless cycle on those tanks.

A fishless cycle with no starter bacteria takes about 3 weeks.
By starting with a little bit of filter media that has some bacteria it will go faster. How much faster depends on how much bacteria you add.
You can also add bacteria from a bottle. Look for Nitrospira.
Keep on feeding the bacteria (from filter media or from a bottle) with ammonia until you are ready for fish. Then a big water change to get rid of the nitrates.

There are quite a few species of algae, and each seems to have optimum conditions, and will need to be identified to be controlled. Basically thriving plants will reduce the algae growth.
Have a look in the Algae forum for more details.