As for how many people will be in your photos, a small venue would be perfect for a small amount of people, but always keep in mind that your shoot should take place in a location that has plenty of room to move and different choices for different looks.
Places with full sun and preferably lots of sky is best for taking images of just your horse, but for horse/person portraits, shade is preferred, though not required. It's usually best to take conformation shots of your horse on well-manicured grass or on another flat, clean surface with a clean background with a lot of sky and no distractions. For horse/human portraits, long grass and trees are great! Inside the barn works great, too, but just make sure it is clear of clutter. The location should allow plenty of range so we can shoot far away using the 200 mm lens.
I love cloudy days and even days with a slight drizzle, so please do not reschedule because of such conditions. When scheduling always consider the weather and have a backup plan if rescheduling is not an option. If the weather plans for wind, does the location have a spot that would provide a good wind-block? In the summer, the morning is cooler and when we have very hot days, I will not do outdoor shoots any other time of day. In the winter, mornings may be chilly, which I love, but dress accordingly.
Remember you, your family, and your pets are the main characters of the shoot, and, in most cases, the background is not. Depending on the number of subjects, I will not need much background to make a good image. The light source is more important sometimes.
Your Checklist Summary:
1) Location: style, travel, weather, clutter
2) Coordinating Clothing
6) Main subjects
7) Consult with your photographer