I can't imagine a good reason not to plant tomato near turnip, unless your particular variety of tomato is prone to the same disease issues that turnips might have. I've never had disease issues with turnips but I have had insects eating holes in the leaves. I've never had pest issues with tomato but I have had alot of disease with them. I've never grown the two close together but I wouldn't hesitate to do it. I would just make sure the turnips are on the south side of the tomatoes to get enough sun.
You do want to avoid planting tomatoes with potatoes, peppers, and eggplant because they tend to share the same disease and pest issues.
That's what I think, anyway