Mix them is fine, it does not really matter in my opinion. What does matter is how much seed (mixed or not) you put on the plot. Ask the feed store or look on the bag how much per acre. If you mix, then you need to account for that. Basically, don't put 1/4 worth ofrye and 1/4 acre worth of oats onto a 1/4 acre plot. Too much seed will result in poor growth - too crowded, etc.

Its very important to kill the weeds and grass too before you plant. Good luck.