Is it necessary to go to ER for a possible broken rib?

Not emergent. If you think it's just a bruise, you will probably be ok, but if you think it might be more, you should get it looked at.
No. Just rest needed ... Unless shortness of breath, then may need to get chest x-ray to make sure broken rib did not puncture and collapse your lung.
That Depends. No doctor is going to say conclusively no to that in my opinion. However, most broken ribs are treated conservatively with pain medication and avoidance of aggravating factors. The problem is that a severly displaced rib fracture could compromise lung function. Fortunately, most rib fractures just hurt, a lot.