If you don't care about strength, sure you could. All the resin does is hold the glass fiber together. The glass fiber is what actually provides the strength. Regular cloth stretches too much to add anything of benefit. It's OK for making the basic shape but it needs to be reinforced with glass.basically. as long as the fiberglass resin is soaked into ANYTHING and enough hardner is added previously. it will work. dont be too worried about the perfect mix, or the kind of fiberglass, hell. if you needed one more layer and there was no more fiberglass to be found, you could rip up a t-shirt and use that.
True, a high powered sub system will exert more pressure and needs to be stiffer to do its job properly.dont be too concerned, you arent making a racing body for a dragracer =)
Use the intended material. If it were as simple and cheap as just using t-shirt, there would be no glass matt or glass cloth or biaxial.you should be fine.