I am assuming you are talking about 4e and the martial arts listed in Arsenal...
This really depends on your GM. Some GMs allow you to use the damage increases in the martial arts section AND hardliners..... Some do not (especially if it doesn't make sense for the style. If the style is predominately kicking like cappoella, hardliners doesn't really make sense

)
Also note that in the arsenal errata they mention that only one damage increase from martial arts ever applies, regardless of how many bonuses or styles you know.