After the Ring-a-Ding-Ding quest do you have to chose to follow only one factions quest line or can you complete different faction's quests? Thx
You have to follow and complete one faction per playthrough as certain factions questlines will counteract another factions questline.
It isn't quite as strict as that but he is right. You can do maybe one or two quests for one faction before swapping but after one or two (it could be more but it isn't many more) you're locked in to that questline.
The Caesar's legion quest breaks off from the others rather soon, but you can go through quite a bit with NCR, Yes man, and House before having to decide between them.