Rosa, Opal, Emerald, and Houda have limited clothing options. Yes, they can fit the Get Set dolls' pants. So that would be a pair of black nylon pull-on pants. If, you have Get Set doll pants, which most collectors probably won't have because the Get Set dolls did not have a long production run.
Yes, the Mixis can wear Ken pants, but the lines are wrong for them. The Ken pants have straight lines; the Mixis have curvy bodies. Possibly the gold harem pants of the Arabian Nights Ken could work. Regular Ken pants though look as though the girls are borrowing their boyfriends' pants.
The Mixis can - just - fit the Get Real Girls' tops and some of the GRG jackets. But the GRG pants are too narrow.
Welcome to the Jakks Pacific doll closet, specifically for their Girl F.O.R.C.E. line. Bingo! The pants fit the Mixis. While Jakks Pacific did not produce a clothing line for that doll line, there were at least 13 Girl F.O.R.C.E. dolls produced. Three dolls for Josie and the Pussycats (Kmart), six dolls for the Charlie's Angels series, and various Girl F.O.R.C.E. dolls, Creanna, Bianca, Lola, and Jaz.
Note 1: the black pants for Valerie of Josie and the Pussycats stain the doll's body.
Note 2: Jakks Pacific produced two other fashion dolls, Catalina and Damita, around 2005. These dolls' figures are closer to swimsuit Barbies' figures. The three pants shown on the box for this series would probably NOT fit the Mixis dolls.
For photos and comments comparing the Mixis with other playscale dolls see this entry.
Opal in My Scene guy top and Jakks Pacific pants; Emerald in Get Real Girl top and Ken shorts
Second view of Opal in My Scene guy top and Jakks Pacific pants