String Gauge question

I'm back with string gauge questions for my 15.75" scale length Kent 744. (I like this instrument, but I'm still unclear what the point of it is, other than it looks cool - my cheap acoustic mando with a single coil sounds better). Its got a slight issue of the wound strings buzzing a little bit on the zero fret when played open, so before I get the nut slots enlarged to fix this, should I reconsider string gauges? Right now its got .040 to .011's tuned to GDAE. Can someone explain to me why I might be better off with either lighter or heavier strings?

Rather long, as it is, 15.75" scale you can go with a heavier string and tune down, say to E,B,C#,F#..

or down to a 4th to a mandola like C,G,D,A..

if you want G,D,A,E, thinner is better because its 1.6" longer than a normal mandolin scale.