the guy told me that you cant fill a nitrogen tank with co2 because of the hose or something. but can i buy a nitrogen tank for beergas and a co2 tank, and then use the same regulator on each one? switch it between the 2
The best solution is to talk with the gas supplier you plan to use. They will know what gases they can put into what cylinders. You will have to use a regulator with a proprietary CGA fitting and unless you want to swap the inlet fitting, you need both bottles to have the same fitting.
Additionally, they may have better prices on bottles and/or regulators as well.
Higher pressure for Nitrogen/BeerGas. Requires different regulator (I believe it's threaded the opposite direction to prevent mix-ups). Check NorthernBrewer, MoreBeer, etc. since I think the descriptions go into some of the detail.