Hello there, i'm going to tune my guitar a half step down from standard tuning.
But i have 0.09 gauge strings, will it sound good with so light strings or should i change to more heavy strings?
It should be fine, maybe .10s would be better. I've played Eb with .09s and it's not THAT bad.

It's all preference, try it out yourself first
.Fender Classic Series '70s Stratocaster
.Peavey Classic 30
.Xotic BB Preamp
.MXR Carbon Copy Analog Delay
.Vox V847A Wah
.Boss RC-2 Loop Station
.Washburn WI-24 (first guitar )
i play a half step down with .09s they sound fine. its all a matter of opinion. but i like them that way.
Half step down with 9's is fine, a lot of people use that setup. I switch mine between and standard and 1/2 step down and use 9's
2003 Music Man Axis Pacific Blue Burst
shud be fine. anywhere after D standard is when i usually change guages

ESP F104 Bass
Spector Legend 5 Classic Bass
ESP B206SM Bass
Axl Mayhem Fireaxe Guitar
Gallien-Krueger 400RB-III Combo Bass Amp
Gallien-Krueger 115SBX Bass Cabinet