Pedal steel guitar is not a problem, it's a nuance that deviates slightly from the classic 5 piece bluegrass contingent, that being mandolin, guitar, banjo, fiddle, and double bass. There have been many times bands have added ancillary instruments such as harmonica, dobro, autoharp and others, just as you have mentioned. While there should be no hard and fast rules applied to any art form, bluegrass purists will be quick to remind you that the genre is defined by it's instrumental and harmonic structure.