Um...you're a private cook, but not a professional? Do you get paid?
So while it's certainly a diversified app menu, I wouldnt say they're not
appropriate for apps--depends how they're presented. For instance if you wrap the
prosciutto around the cantaloupe, maybe top with a cherry tomato, and slide onto a
frilly toothpick, it makes a very pretty app.
The cucumber/feta/tomato you've listed as a salad, but can be converted to an app in
a few ways, such as hollowing out cucumber slices, filling them with feta/tomato chunks,
maybe a touch of basil for nose, taste and color, topping with a halved plum tomato, etc.
Pretty AND healthy.
Dessert: turnovers....or perhaps smaller, filled sweet epanadas, with or without
something gooey and colorful drizzled over them.
As to pricing, I take it this is outside your "family" job... for a third party?
Food cost, overhead, your labor, (including planning, shopping and client consults)
hired labor, and your desired profit.
You didn't ask about quantities so presumably you have that figured.
The problem with doing only 20 people is you need to have a minimum established.
Mine's always about 50ppl on app-events. And that's what's charged,
be it 20, 30 , 40 or 50 people. You reduce it according to only 20 and you're going
to cheat yourself.