Introducing formula will effect your supply if you don't express. Your milk supply works on supply and demand - if baby sucks, then milk is produced, if baby doesn't, your body thinks it doesn't require it, so reduces the amount produced.
A one off bottle of formula might not effect it too much, but a regular one will.
What are your reasons for introducing a bottle of formula at night?
Lots of people successfully offer one bottle per day from this sort of age. I think as long as it's consistent ie the same time, the same amount of milk offered etc your supply ought to adjust, as it would if your baby starting sleeping through.
I second the question, why are you wanting to introduce a bottle? Lots of parents feel some duty to introduce one due to fears of bottle refusing babies and it creates lots of extra work for little gain in some families.
I'm just worried that if I decide to wean him at some point he won't take a bottle. I may not want to wean him and just let him go right on to cows milk in a cup. But he's only little and I may feel differently in a few months.