Casting has to match the gender of the character, besides that anything goes.
00 - Kreia
11 - Atton
22 - Bao-Dur
33 - Visas
44 - Handmaiden
55 - Mira
66 - Mandalore
77 - Scion
88 - Nihlus
99 - HK-47
00 - Goto
Trips - Exile
The role of T3-M4 has already been filled by Eric Roberts