I love the fact you are putting a lot of thought into this treat for your sister and for all the people attending the party, and yes, I understand the concept of making a large cupcake. Are you doing it 2D whereby they would all be laid flat on the table to form the large cupcake or are you doing it 3D?
If you want to cut a hole in the cupcakes you can use what is known as a cupcake corer, these are very cheap and can be bought from a cake shop, Amazon or eBay etc.; you can also use an apple corer too if you wanted. It looks like this; cylindrical with a serrated bottom which is inserted into the cake...
1.
Basically, all you do is hold it under the frilled edge with your index finger and thumb, press it into your cupcake and then start rotating it in a back and forth motion until you reach your desired depth, if you look at the green arrows you will see lines on the corer, these indicate how much of your cake you are removing. Once your desired line on the corer lines up with the top of your cupcake stop rotating.
2.
To remove the cored centre simply rotate slightly upwards and pull the corer out slowly and it will pull the cake with it leaving a hole in the middle of the cupcake.
3.
This is where you would add your desired filling either your chunk of cheesecake, just the cheesecake topper (cream cheese & sugar - alternative cheesecake toppings can also include a little vanilla essence, cream or condensed milk too).
4.
Then when you have added your filling place the piece of cupcake back in the centre by pushing the spring section in the middle of the corer. You will find that you have to trim a little of your centre cupcake piece as your filling has taken it's space, therefore, push down slowly on the corer just so the cored cake is showing, slice along the end of the corer to remove a little of the cake and then place back over your cupcake and push in fully, thus hiding your filling.
5.
This is what your cored cake will look like when it is finished.
I would use a corer as that way you know that each cake will have the same amount of filling and you won't end up with broken cupcakes after hacking into them with a knife. I'm not saying that you will it's just that sometimes cupcakes can crumble and I don't want that to happen although if it did you can make cake pops with it so it wouldn't matter!
With regards to using buttercream on the red velvet cakes that's fine, buttercream holds its shape better, and you can pipe much easier with it.
My only concern for you is that you don't add too many flavours to the cupcakes, I'm not sure what other flavour cupcakes you are doing, maybe do some plain Victoria or Vanilla Cupcakes as a contrast to your red velvet. I know you may buy the pre-made mix ones, but if you get chance I have this Victoria Cupcake Tutorial, these are really easy to make and do taste delicious.
Here is your step-by-step guide to making beautiful Victoria Cupcakes. Preheat the oven to one of the following: Fan assisted: 180°C Regular Oven: 190-200°C Fahrenheit: 356F Gas Mark 4. Equipment: Mixing bowl Electric whisk Spatula Muffin Tin, Muffin cases. Ingredients: 150g or 6oz...
www.helpmebake.com
You're most welcome on the help; I hope the above makes sense if it doesn't let me know.
I bet they'll be great when they are done.