Constructing decoder 5/32 using smaller ones, without enable
I'd suggest working backwards from the output.  Since you want only one output active at a time, and because you don't have an Enable on your devices, one simple way to approach it would be to use only a subset of outputs.  

For instance, if you did that with a 3-8 decoder, you might use only the low 4 output bits and simply not use the upper 4.  The effect is that for any three-bit input, the device would only provide a (connected) output if the high bit is zero.  If you work backwards to the input from there, you can come up with a mechanism that will work.