Most hotels charge per-room - sort of - but are strict on how many people the room is for. The price goes up for different sizes. A single is cheaper than a double. If you get a double you can’t just put a third person in it. You need to get a triple room. Some hotels will bring in a bed to make a double into a triple and charge for it. Others have specific triple rooms. Some hotels don’t do either, there is simply no way to have 3 people in one room.
Allergies - print out a card in the local language to explain. Ask your hotel ahead of time if it has a free breakfast and if so if it can accommodate your allergies. If not you would need to figure out another solution for breakfast.
I personally prefer apartment with allergies. Most of the time restaurants work fine but it’s good to have a fridge and place to prepare snacks that I know are ok. I don’t “cook” on vacation very much, but I do buy cheese, yogurt, gluten free bread and cereal, milk, etc for snacks and in case of emergency.
Even in a hotel you can stop at a grocery and buy shelf- stable snacks like fruit or snack bars.