I think the mk4 polo seats fit straight in, but imagine the mk7 ones shouldnt be too bad, you could just buy new subframes if not
 
MK3 Ibizas accept MK4 & MK5 Polo seats, whereas the MK4 Ibiza can accept MK6 & MK7 Polo seats.

You'll need to mod the subframes to get 'em in, by quite a lot!
 
The subframe is similar to the TT ones, but not the same. The seats bolt to the floor at the side whereas the MK3 seats have one bolt and guides in the floorpan.