WDW considers a 'wheelchair party' to be 5 plus the person with a disability.
For some things, they are able to allow one or 2 extra, but for others, they can't. For example, the wheelchair accessible boats on Small World and at Mexico holds a total of 7 people - one wheelchair and 6 seated guests.
The ride cars for Toy Story Mania at the accessible boarding area hold a total of 6 guests (6 seated guests or 5 seated guests and one who remains in the wheelchair).
So, if you need to split, it will usually be because of space considerations. Sometimes it's the number of guests who can fit in the ride car and sometimes it's the amount of space in the waiting/boarding area. So, plan that you may have to split into a party of no more than 6 and decide who will be going with who.
For Illuminations and the MK night parade and all the daytime parades, the handicapped viewing areas become quite crowded. To enable the largest number of guests with disabilities to see, the CMs line the wheelchairs and
ECVs up in a row along the curb, nearly touching the next wheelchair/
ECV. The rest of your party will need to stand behind the wheelchair/ECV. So, at time when it is REALLY busy, they may limit the number to one or 2 companions for each wheelchair/ECV.
For Illuminations, since she can park at the fence and no one will be in front of her, we have found it is generally just as good to find a spot along the railing. The wheelchair viewing area at the entrance to World Showcase is usually VERY crowded (like to the point that DH and I were kind of squished behind DD's wheelchair and had blocked views - and we are not big people). There are other wheelchair viewing spots, but they have trees just in front of the viewing area, so many people do not feel they give the best view.
For Fantasmic, the wheelchair seating is in the back row, where they have spots for parking wheelchairs/ECVs. The 2 rows in front of that are for the rest of the party. For a party of 7, you will probably fit, but may be asked to split if they need the room.