I am making letters out of Flood Icing or maybe pastillage. Either way, my intention is to "glue" the letters onto the sides of a fondant covered cake. The letters however will not have 100% surface contact with the cake, meaning part of the letter may be off the cake. What "glue" should I use? I need this to set fast as I don't have too much time to glue letters and support them until they dry before cake is to be ready.

by flood icing do you mean royal? If so I'd save a little bit of it, and use that as the glue.

If you can just tape 2 cake pans, bottom sides together. I would put the letters taped to the sides and then flood. I use color flow to do these type of things but you could even do this with fondant. As they dry they will confirm to the curve of the pans.
I hope this has helped you.
evelyn

Flood Icing - yes, royal icing thinned out with egg white.

I have use royal icing before to glue things on but unless it is supported for a little while and allowed to dry the object can fall. Seeing how the letters will only have a couple points of contact with cake where they can be glued I need something that dries faster.

Melted candy chocolate works great when attaching items to sides weather it's fondant or buttercream.

I just put a toothpick in a discreet spot to hold up the letters/decoration until it dries. Usually only takes a few minutes and if you angle it no one will ever see the tiny hole, not even you.

Unless the letters are big and heavy holding them in place for a few seconds should work.