This may not be helpful to you but I have mixed frame rates etc. in Vegas before with no issues. One tip I heard but not tested by me was to put make sure you keep the different formats on different tracks. Whether thats true or not I don't know. A work around if you really can't solve this would be to put all the 60fps footage on the time line and render it out at 50fps. Then use that for your project.
I hope some of this is useful to you, it's all just "internet stuff" that I've not tested myself.