What things to watch for when installing sewer line in the basement under 15" concrete slab
Use 4&quot; - don't cheap out and use 3&quot; because you can technically get away with it. That will instantly limit likely problems, and costs very little to do up front. Small lines (sink drain, vents, buried traps) are required to be at least 2&quot; diameter under concrete IIRC.
Unless you have some absurdly torturous path in mind, a cleanout every 50 feet is generally adequate. If you are going to make absurd corners, more cleanouts may be advisable, but not making absurd corners is more advisable. You certainly don't need one every 10 feet, and providing the required access to them every 10 feet might screw up your basement plans anyway.
Remember to allow for future finish when roughing in your powder room access - we've had a few questions come by where the fixture rough-in was spaced from the unfinished wall, and there wasn't enough space when the wall was finished and a few inches were lost in the finishing. Also remember that vents are needed as well as drains. Much easier to put them in from the start than to try and retrofit after concrete is poured.