1) It's safe, but you need to be a bit more careful about form/technique. You can pretend that there is an imaginary bar that keeps the dumb-bells at a constant distance apart as you do the exercise.
2a) Bent-over Row with dumb-bell
- Keeping good form is a little more difficult.
- You activate the secondary stabilizing muscle groups more (which is good).
- As your right and left side can't help each other as you are working out, you should be able to lift less weight and/or do fewer reps than with a barbell.
- mass gain may be slower, at least initially.
- I like it
2b) Bent-over Row with barbell
- Keeping good form is a little easier
- Because the right and the left side help each other you can lift heavier weights, at least initially, this can cause a faster mass gain.
- I like it less
2c) With a bench it's easier, but you can also do Kroc Rows without a bench.
You can hold on to a chair or table to help you stay balanced
Kroc Rows with Matt Kroczaleski
You can also do them just resting one elbow on your leg. This is the only video I could find, the technique is poor (way too much upper body movement and hard pulling), but I hope you get the idea.
33x42 kg kroc rows
Keep also in mind that Kroc Rows are an asymmetric exercise. The risk of injury is a bit higher than with other exercises because your core is compensating for the weight on one side. This works also your core but could also strain your back muscles too much.