ORIGINAL: ED STEFAN
Using slightly less hardner will slow down the cure rate. In fact, when I'm mixing supposedly equal lines of resin and hardner, I err for less hardner than resin. I wouldn't go so far as to say 1 part resin and 3/4 part hardner, but slightly less hardner will not compromise the final cure.
I read on at least one epoxy manufacturer's web site that you should not do this. Can I find the link now? Of course not. I always keep a variety of epoxies in the shop and choose one with the appropriate cure time.
Edit: Found it
http://www.wessex-resins.com/westsystem/wsmixing.html