Small oven, 20 people. Need roast potatoes hot and crispy so can I serve turkey at room temp after cooking earlier and then putting roast potatoes in?
Sure, there is absolutely no problem with your strategy. The turkey will continue to cook when it is out of the oven, and it will have to time to reabsorb juices and be easier to carve if you let it stand. Then your oven is free to roast potatoes and heat or reheat other sides. It sounds like you have a good game plan.
That'll work and your turkey won't be "room temperature." It really needs to rest 10-15 minutes anyhow before slicing (a task that easily takes ten minutes itself). As mentioned by E E Faris, the bird will continue to cook a bit even after it is removed from the oven.
By that time, your potatoes will be halfway baked, so everything will hit the table warm.
Just tent the bird with aluminum foil, then cover the dish with the sliced meat with foil. It'll stay warm. You can pop the dish into the oven if you want to warm it up a bit, but I generally find that unnecessary.
Normally, I would aim to have the bird done about 45-60 minutes before dinnertime, and start slicing it 20-30 minutes before serving.
You don't need to have the entire turkey sliced before dinner (or all the potatoes cooked). You just need to have enough for the first serving. You can roast a second batch of potatoes after people have sat down while carving the rest of the turkey for those who want second (or third) trips while people are helping themselves to their first round.
A lot of the success of your meal will be based on your ability to prioritize tasks, break them up into manageable chunks, and properly delegate the right tasks to any kitchen helpers you have.
Yes! In fact, I just had a conversation about exactly this the other day - having cooked 20+ Thanksgiving dinners, for groups ranging from 6 to 15 guests - and no matter how well I strategize timing, oven temps, space, etc. - by the time all else is warmed/cooked and the turkey is carved and served, it's never piping hot anyway. Serve it with hot gravy, on a heated platter if possible, and it will be fine. (Disclaimer: military precision on these matters isn't my forte - having a relaxed, delicious, convivial celebration is way more important to me than ideal turkey temp.)