You just need to use negative values in the Add field. No need for another action for that.
True, although if you need to subtract from a
variable you'll need two actions - one to multiply the variable value by -1 and a second one to add that result to the intended variable. In that case an actual subtract float action would be handy.
Sounds like a great exercise into creating a custom action for whoever wants to get into the most powerful aspect of playmaker, after all it'll be just one signal switch. C'mon guys, try it, it won't bite you!