You certainly can have liposuction 6 months after a tummy tuck provided that you have healed and the swelling has dissipated. Best to go for a consultation.
Liposuction is often needed in addition to a tummy tuck to achieve more defined contour. Sometimes the procedures are combined and other times they are separated. If separated, it is best to have minimal post-operative swelling before proceed with the second procedure. Six months should be ample time, but every person is different and one should rely on their surgeon's judgement.