custom tiles

[perkeyone]

so i was considering making custom tiles for my map
i know that the normal texture space allowed in a map is 13
and i can get 16 different tiles if i use triggers
(im not sure about other ways to get more tiles)

i plan on making a 40 floor tower/dungeon
and i was going to try to make each floor look different
each floor i was hoping to get at least one tile type for the rooms of the dungeon and one tile for the halls

so i looked at an existing tile in paint and figured out how it works
it is a 256 by 512 square and takes up 2 texture space.
it has 18 floor tiles and 14 edge tiles

i figure if i make each floor use a single variation of the tile i can squeeze the amount of tiles i want out of it.
i can take the 18 floor tiles and make one floor and one hall tile for 9 floors
and i can get at least 6 tiles with 2 texture space to fit in my map
so i can do up to 54 floors this way

the problem is i wont have an edge part to match all the variations.
if i do implement this what should i do about the edge tiles?
should i make them black? white? grey? clear?
and how would that look?
will one tile look bad?
if 1 is not enough how many would be better?

if this is confusing i can elaborate and make a picture to illustrate it.