A 12" on an EQ6 would definitely be pushing it weight wise. You have to consider that you will be adding Guide scope, camera gear etc as well which can add more than you suspect. The recommendation for astrophotography is to try to stay around half the rated carrying capacity of the mount.
I've got a 10" Newt on my EQ6 but I rebuilt it as a a lightweight Serrurier Truss of around 11 kg. Nonetheless with camera, finders, guide scope etc it now totals about 18 kg on the mount. Marginal but as the Serrurier design requires minimal counterbalancing it seems to be working.
I have bad news for you, you will always be buying stuff for this hobby ...