an EQ5 might just take a 10" but certainly not the 12", l'm assuming a Meade OTA, a 9.25C would be about the limit for visual observation but probably a bit big for good photography. the EQ6 would be a better bet.
Leon, it occurred to me that your newt may be no ordinary newt, so I checked your previous posts and I see it was made from old battleship parts Maybe you could add weight to the refractor.
no way for the 12, its a big scope, the 10 would be pushing it uphill to I think! i am going to be putting the 12" SCt ota i have on the eq6, thats 18kg upfront!