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It is possible to over train with anything. The main thing is not to get burnt out or injured.
I personally wouldn't do cardio everyday. As far as "is more cardio unhealthy?", if you have a crazy heart rate or something, it can be deadly depending on your intensity.
The exercise that you are doing with the dumbbells is for shoulders. You can make should defined but it's hard to make them very strong. The 10's would probably last you for quite some time, and you'll probably never need any where near the 25's for that evercise. There are many only exercises that you can use the 25's on so don't just let them collect dust!