I think you mean how effective are " Dumbells " instead of using the barbell on your shoulder. Umm.. well your best bet would to use a barbell when squating or deadlifting... because you need a barbell to perform these movements! If you wanna use dumbbells and like hold them at your side if its more comfortible -- I'd suggest you do lunges. These are great for your glutes and hams.
And for the second question -- Yes, your still working the muscle. Just concentrate on holding the contraction and more importantly contracting + flexing your glutes.
P.S - You're a damn good lookin girl.