Hi Tricia! I went through the exact same dilemna before I got Emmy. I think it all depends on Lexie's personality. Is she dominant or jealous when other dogs are around are is she more of a submissive sweeter personality? Mia has always been submissive and ultra friendly toward other dogs, so another female worked well with her. However my mom's yorkie Peaches is very "bossy" and dominant and would probably not do well with another female, however does well with their male king charles spaniel because boys tend to be really sweet and let the girl push them around. Having Emmy has made Mia more assertive
but they compliment one another so it works.
Hope that helps!