Yes, you can use Red Focus at night time. Since Red Focus does not contain any stimulants it should not prevent you from falling asleep. We wouldn't recommend taking it right before bed, but for example, if you were to take it at 8 PM to crank out a few hours of work before you go to bed at 11 PM – 12 AM, you shouldn't have any trouble falling asleep.
Yes, you can mix Red Focus with a lot of other nootropics for a synergistic (boosting) effect. That's the great thing about nootropics, there is a lot of variety that you can add to stacks in a healthy & safe way. However, Red Focus is a powerful stack in its own right, containing 10 nootropics so I'm not really sure what you would want to add there.
We wouldn't really recommend taking L-Theanine with Red Focus since L-Theanine is one of the ingredients already included in Red Focus!