Re vignetting: many people use a 35 mm sensor with the RC10, so your scope is fine. The likely culprits would be either your focal reducer doesn't provide a large enough illuminated image circle (a 2" nosepiece barely illuminates a full 35 mm sensor; I think a 0.5x 2" reducer would vignette severely?) or your T-ring/nosepiece adapters are too small (many scopes use M48 attachments for larger sensors, instead of the standard M42 size).
As for OAGs, there are many brands available on the market. However, if you want to use one with a reducer/flattener then you'll probably need to minimise the amount of back focus between the reducer and your camera. Here's one that's very low profile and can be used with full-frame DSLRs:
http://www.teleskop-express.de/shop/...-fuer-CCD.html
The corner stars will be horrible on your 6D without a flattener/reducer (there have been many posts by RC10 owners on here about it). Some people choose to crop out the corners - which reduces your field of view - and others use flatteners/reducers such as the Astro-Physics AP27TVPH which are designed for larger chips.