Are you saying your Porta II is overloaded with those scopes? That's odd, as I sometimes use a Meade 6" SCT (5.0kg) on mine, and it's near the limit but basically fine - vibrations are under a second.
The ETX125 is ~3.3kgs and the C6 is ~3.6kgs IIRC, I would imagine the Porta II can handle these just fine
What Jon said. Both scopes were "wobbly" once loaded up with 2" diagonal, 2" eps, 9x50 RA finder and dew shield. I never tried actually using the 125 with the 2-speed focuser as it was obviously never going to balance properly just setting it up for the picture. The Vixen Universal Plate also weighs 1lb 6oz which has to be included in the weight of the load. I used that to have the focus knobs on the side, not the top or bottom. Never had either settle within a second. That's why I got the A-T which had no problem with either. It was fine with the Vixen 110.
My personal method now is to have a mount rated for at least twice the weight of my fully loaded scope. Which includes a dslr. I may never attach one but if I do I want to make sure it will carry it with ease.