Well, don't freak out. You haven't even had a dishwasher walk out on a Friday night yet. Then you can freak out.
Not having enough capacity in an FEC500 would be a good thing. Business would be doing great!
Scheduling is going to be very important. I'm thinking brisket in at 8pm, butts in at 10pm. Ribs in at 8am, rib tips and sausage in at 9am. I don't know about chicken. Maybe in at 9am when you pull your butts and hopefully your brisket. You really don't want it dripping on anything else, so it'll need it's own side of the smoker.
You could always buy a smaller CS, maybe an Amerique, to do rib tips and sausage, or just use it for chicken. You could cook a mess of rib tips and sausage in an Amerique. But don't do anything until you see how things go. It's okay to run out of some things because it shows people you're serious about serving quality food. It can be turned into a plus if handled correctly.
If you have a holding oven/cabinet, you'll be okay unless you hit the ground running flat out. Even then, you could cook a heavier load of brisket and butts overnight, then load the FEC up with a heavy load of chicken and ribs in the morning when you pull your night load, then follow up with another rib and chic load in the afternoon. FWIW, I think ribs hold well for a couple of days if you take care not to let them dry out and finish them on a grill. May not be what you had in mind exactly, but you can do a good product like that. There is a way. You'll figure it out once your customers develop an ordering pattern.
You will need to cool and hold some meat overnight even if they're just leftovers. I would use leftover chicken in brunswick stew or chic salad. I would mix any pork at least 50/50 with fresh before I served it. I would use left over brisket for burnt ends or chopped BBQ beef. All this is just my opinion as I don't like to get day old food from a restaurant if I can tell it's day old food.
I would also create a little frozen reserve for those "just-in-case" kind of events. Frozen pulled pork and chopped beef in quart and half gallon containers holds well, can be rotated by using it in stew and beans, and sold to people in bulk also. You might be surprised how many people would prefer frozen containers instead of fresh. A quart will hold 2#+, and a half gallon will go just to 5#. Very useful sizes to have on hand. Keeping 50-100# available won't hurt you at all. Here, people get frozen to take to the beach or mountains. I don't know what the heck people in Ohio do when it's not football season, so your needs may differ. Maybe they'll eat it as they drive back and forth between the football hall of fame and the rock hall of fame? I don't know. Ohio seems kind of boring now that I think about it.
As to your question, "Am I stretching myself to thin?", well, YES! You are. Physically, emotionally, financially, spiritually. Hey! That's the restaurant business for you!
You'll be fine. Just make sure you can sleep at least one night before opening day.