Filled potato skins
Dips and veggies/breadsticks
Birthday cake of course
Then meat of various types on the BBQ.
I agree with previous posters . . . your menu is quite starchy.
I think the idea for parties like what you're planning is quality/quantity vs. variety.
As a rule, I usually provide at least 2 servings per dish per person invited.
Maybe pick 2 of the first four items on your list. The Colcannon sounds wonderful (from what I looked up on line) can easily, cheaply, serve a lot of people. But I really like the idea of skins -- creative and unique.
I'm thinking one of the potato dishes and then your rice salad for a different type of starch (could you, please, post a recipe?) Or two of the potatoes and nix the rice.
Personally? I loveses crusty bread.
Salad -- I would go big with a fresh undressed garden salad. I usually serve a variety of dressings on the side (Vinaigrette, Ranch, French, Bleu Cheese, whatever) so your guests can choose their own flavor. (Oh. And crusty bread goes very nicely with a garden salad.
Quiche and hard boiled eggs . . . I think that I would pick one or the other. Maybe go with the Quiche (will it be mini-one-serving quiches?) because quiche is a bit classier than simple hard boiled eggs . . . but that depends on your guests. What do you think they would they prefer?
Dips and veggies/breadsticks -- I would sway away from the breadsticks and go with a couple of different flavored crackers instead. Will this be your appetizer? Maybe add some cheese and a fruit platter.
Absolutely no need for pizza.
What to cook on the grill? Hamburgers for everyone! if you want to make it easy on yourself.
This is what I usually do. Pick one meat to grill: burgers, brats, chicken, and have plenty available for everyone (1 and a 1/2 or more servings for the number of guests invited.)
But this is a "know your audience" type of thing. The people that I grill for have no dietary issues, so everybody is happy with whatever I serve . . . usually 1 main meat dish.
I wouldn't offer more than 2 grilled choices. Maybe a beef and a chicken.
Again, it's quality/quantity vs. variety . . . no need to knock yourself out by serving all
of your favorite dishes.
Good luck! and Have fun!