6.5 pounds is not heavy. 65 pounds is heavy.

6.5 isn't comparatively heavy, but it may be more than a lot of old work mountings can handle on drywall. The plastic ones with more substantial flip out wings will fare better than the bendy tab metal things.

A toggle bolt or hitting the stud in addition isn't a bad idea.

6 lb is the limit for the old work plastic box. See the last sentence at the last page of

http://www.carlon.com/FAQs/FAQ-ZipBoxes.pdf

I plan to mount the light directly onto the stud without any box. Then I will not worry it would fall down someday. Is there any concern in terms of wiring?

The only concern would be if the fixture is rated as a junction box to allow connections to made within the fixture thus not requireing a junction box.
Does the fixture have knock out where a cable clamp can be installed? If not then a box may be required.

I don't think the fixture is rated as a junction box. It has no knock out. Can I use wire staples to secure the wire on the stud?

All connections MUST be made in a box. If the fixture is not rated as a box then you must install a box. The fixture can still be mounted to the wall for support.

I plan to use a metal pancake outlet box, which can be mounted at the face of 2x4 directly.