An 18" Round cake pan should fit in your oven but to tell for sure measure it! Your oven I mean And yes and yes 2 1/2 18" as the bottom tier would definetly cause stability issues I wouldnt trust it to support anything much. You MIGHT get away with 1 tier but I wouldnt chance it with 4.. its to risky.
Because "standard" ovens are so varying these days, sometimes an 18" will fit and sometimes it won't. I know I can easily put a 16" in mine, but a 18" is pretty tight. To ensure good airflow, I would use the half rounds in mine. If they are leveled and doweled properly, there should not be a stability issue.
I use the 18 in 1/2 rounds. I have never had a stability problem. The fondant cake with gumpaste roses in my photos is a 18,14,10, & 6 in. and the present cake in my photos has a 20 in. cake on the bottom which is made up of 8 - 10 in square cakes. Again I didn't have any stability problems. I try not to put a dowel in where the cakes meet, but other than that I dowel as I would any cake.
Just be sure to rotate the second layer 90 degrees from the seam on the bottom layer, so the layer joints line up like this "+", not lined up straight like this "l" and you you shouldn't have stability problems. Be sure to dowel it to support the tiers above it is all.
I personally have not had good luck with the half pans. I have now done 2 large cakes with the pans (multiple tiers) and had both implode on themselves. This is even with leveling, turning 90 degrees, filling with less than usual amounts of filling, etc. If you try them, I would suggest that be the first tier you assemble in case there are any problems and you need to re-bake.
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