You should be ok, moving the 2 established filters over but it would be a good idea to also move all of the decor(plants, rocks, driftwood, ornaments) over as well until the new tank and sand establishes bb. After about a month, you can start removing the decor you don't want to keep in the new tank.
1 bag of pfs will cover the 125, its just a matter of how deep of a substrate you want. I believe 2 bags of pfs will easily yield 2" depth of substrate in the 125.
In regards to your stock, 25 rummies and cardinals will be just fine, you don't need 50 of each. I would nix the loaches as they may harass your discuss and out compete them for food. I would also not do BN but rather add about 10 ottos to control algae.
Your Finnex lighting is designed for plants, so you will probably have a constant issue with algae, so get a regular LED light will decrease this situation.