You can hang 15kg with brown ( 7 mm bit ) Plugs, but it all depends on how well you can get a fixing in the wall, if it goes into something solid ( brick, block ) then it will be ok , but if it’s a joint or a soft wall ( thermalite). You might struggle.
I would use a 75-100 mm screw and make sure the plug is deep down in the hole, tap the plug in the hole then hand turn the screw in the tap the plug down the hole. If the plug is right down the hole then there is a better chance of a good fix and won’t pull out if hanging something heavy.
It’s all about feel when getting a goodfixing with rawl plugs. Sometimes it’s best to use a hand screwdriver to achieve this so not to over tighten ( snap screw or spin the plug).
I’d probably go with a 10 gauge screw with a brown plug but you can use an 8 or 12 if it feels to loose or to tight in the hole.