A villa near Viareggio or Forte dei marmi: it all depends on your budget. How much are you prepared to spend? Also, most villas nearby the beaches are already booked in August by now. Only villas at about 25-30 km from the beaches may be still available.
Apartments: you can find hotels by the sea that also have nice apartments inside their buildings. Something may be still available. Pool is shared.
As far as beaches in Forte dei Marmi and Viareggio - it is ONE long coastline, one beach at each town... what changes is that the beach is divided up and there are rentals of sun chairs with sun umbrellas along the beach, allotted to different companies, all offering changing rooms, bathrooms, showers, bar or even restaurant where you can eat at... so I'd recommend you head there EARLY and walk along the beach, see which place catches your fancy.
Rental for umbrella and 2 beach chairs generally runs 20-30 euros for the day.
The spots closest to the water go first... so you will still find spots later on but they tend to be further away from the water... there are also parts of the beach that are "free" where there are no chairs or umbrellas, those parts get crowded quick early on... but you have to take your own gear.