Pokemon demo rules:
When you line up four you go into combo mode, then you need to line up three to continue, after that two will suffice. When the counter on the top left runs out (number of tiles to be dropped) you go into combo 2 mode, after it ends you gain points if less than five (or four?) pokemon remain on the field. The less, the more points you get.
I don't know what the three pokemon it shows you at the beginning of each level mean.