Hi Zuzana,
There should be no problem staying at an albergue the night before you start walking, you just need to have your credencial ready to be stamped. The only time I can imagine you having a problem is within the final 100km. Where do you intend to start from.
Staying an extra at most private albergues won't be a problem, a lot of the municipal/parochial one have a one night only policy, though they may make exceptions in case of injury. There is certainly no rule that you can't stay an extra day or so in a given town.
Buen Camino,
Rob.