Substitute for 1/4 cup finely chopped green pepper?

If it calls for just chopped green pepper, cayenne, red pepper flakes, or hot sauces are not a substitute. Green pepper is an extremely mild pepper, with no heat at all - a rating of 0 on the heat scale, or Scoville scale. All the other peppers you mentioned are hot - or a totally different type of pepper (in the same family as ground black pepper). In this case, you can just leave it out.

Now, if the recipe calls for a hot green pepper, like a jalepeno or habanero, substitute hot sauce to taste, reducing the amount of other liquid just a bit so the sauce isn't too runny.

Since the peppers will be a cooked vegetable, try making a salad. Especially if you are making the peppers spicy, a salad would be a nice way to cool down your mouth. Something simple with a dressing that is on the sweeter side, like tomato bacon or french =)

My guess is that the green pepper isn't really added to mac and cheese for flavor, just for color and perhaps some texture. I think you can just cut it out and don't need a substitute.

I would think the sauce would be better just put some in a tablespoon or two and let it cook for a bit then taste it if not hot enough if that is what you are going for add a bit more till it has enough kick for you.

You can just take the same ammount of pepper, and chop it really fine, so you will still get the same flavor, but not the chunk of vegetable.

There really is no substitue for fresh green (bell) peppers. If you don't like it, leave it out - I think it would overpower the mac and cheese flavor

if you like heat then use the ones that you have , if you don't like green peppers omit them,or you could put them in the food processor and make it into a paste and mix it in your Macaroni and cheese recipe