When using Sweets or BBS, do you use a brass jag or nylon/plastic? Was wondering if these solvents would harm the jags and lead to false "copper" showing on the patch.

I usually apply solvent with a brush, not a patch. For a final copper test simply use a nylon brush. Then swob out with a patch on your brass jag. Any colouring showing on the outside of the patch will definetely be from your barrel. A little blue on the inside may indeed come from the jag.