I don't know about the piping gel but it sounds like a good idea to me! I think my FBCT icing was too thick and that is why the 'lines' where the icing was piped was visible so I would go for a bit thinner next time.
Yes, you should use a med. icing,but be cautious that you do not get it too thin. The tip you use just depends on you. I just use a small parchment bag with just the smallest hole (= to a #2 or #1 tip) for outlining, then for filling in it just depends on the size of the area to be filled. I also freeze the transfer after outlining, and after every new color added, because it makes it easier to manuver. If after you put the transfer on the cake you notice there are little lines, just use your finger to smooth them out. For areas where it is too small to use your finger because you would smudge colors together, just get that color of icing out (you should have kept them in the bags in a tightly sealed container just in case of any oopsies) and smooth just a dab to fill in where needed. Just wanted to throw that out there. Good Luck!
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