Can I use an aloe vera gel directly from the plant overnight, even if I don't have any acne.?

Yes. It is very soothing. However, be sure to dilute it - the plant form is concentrated.
Yes. Aloe vera gel is soothing and healing to the skin, whether you have acne or not. See http://www.Naturalnews.Com/031687_aloe_vera_skin_care.Html.