Just to be sure: I've got a Paterson System 4 tank that mentions on the bottom "takes 35mm film or 126 films". I've used the reel for 35mm film, but it seems I can make it bigger too. Would this tank + reel work for 120mm film too? Or should I buy another tank.
Depends on the tank you have. The smallest one only takes the reel when it is set to 35 mm or 126. All the bigger ones are able do 120 film too. The test is easy: put the reel in 120 mode and try if it fits. If it does you're set and done, otherwise you have to buy a bigger one. These larger tanks take 2 reels (or more) for 35 mm by the way.