Railcraft currently has Tin and Copper, adding Bronze Alloy will be trivial to do as a result.

This could be used in some crafting recipes. I'm thinking definitely a component for the Manual Rolling Machine ( #1125 ).

probably 1/5 tin to copper ratio or some different ratios.

but crafting.. how to do so? Forestry way or we have to have a grinder to grind then down?

usuall ratio is 1 tin to 3 copper for forestry.

1 Tin + 3 Copper is the current plan, yes. This is for consistency within other mods like Forestry, IC2, GregTech, and Thermal Expansion.

As for grinding requirements: That is being discussed currently. Very likely the Forestry Method will be the alternative recipe if you do not have machines for example if there is going to be crushing or smelting involved.

I'll be working on the implementation of this.