Matching reel and line/rod weights. What can one get away with?

I am still baffled by the whole line/rod/reel weight thing. I have a 7
weight Hardy JLH Golden reel winging its way towards me from England to use
on my old Fenwick 7 weight glass fibre rod. I have also decided to look for
a cheap multi-piece travel rod to allow me to fish when hiking, and it would
be nice if I could use the same reel. So, what is the lightest rod I could
get away with using the Hardy reel on? I know one has to match line weight
and rod weight, but does one absolutely have to match the reel to the rod
and line? Since everyone seems to say that 4, 5 and 6 weight rods are right
for trout, I was thinking that maybe I could get a 5 weight rod but only if
I can use the 7 weight reel on it.

Dan (who is beginning to think about making a trip to Sacramento just to
talk to Bill Kiene in person.)