In case of a 3 Axis Gimbal there is no problem with the existing landing gear. You only need to replace the CFK tubes, the rest of landing gear can remain on the frame, its also necessary for stability of the frame. in this case you need a seperate rotating landing gear.
Basically the maximum payload depends on thrust of your engines. FerdinandK has tested that with 4,5kg payload. See here: http://vimeo.com/65656686 . I can't tell you the exact maximum payload, but I´m sure that an Eos 5D or Red with gimbal won't be a problem for the frame. You should search for proper engines. The T-MOTOR MN4014-11 why not? I'm interested to see this motors on your frame. This motor should be fine for the frame. Changing the outer diameter of Booms is not possible, but if you don't trust the tubes, you can use smaler inner diameter. Actually it's 16x13mm. I think, that is strong enough. I think with your motors you've choosen, you are able to pull weights more than 10Kg! The next step is to rent a helicopter...
Basically the maximum payload depends on thrust of your engines. FerdinandK has tested that with 4,5kg payload. See here: http://vimeo.com/65656686 . I can't tell you the exact maximum payload, but I´m sure that an Eos 5D or Red with gimbal won't be a problem for the frame. You should search for proper engines. The T-MOTOR MN4014-11 why not? I'm interested to see this motors on your frame. This motor should be fine for the frame. Changing the outer diameter of Booms is not possible, but if you don't trust the tubes, you can use smaler inner diameter. Actually it's 16x13mm. I think, that is strong enough. I think with your motors you've choosen, you are able to pull weights more than 10Kg! The next step is to rent a helicopter...