Hi, at 4 months they should be tender enough, although you would be doing well to get much meat on a LS at that age, how long did you leave the carcass to rest for? I normally pluck straight after dispatch then hang overnight and gut the next day.
As for the vent, just cut it out bigger no harm done.
The breed will be down to personal preference but here are a few things to consider,
are you looking to rear and process all birds? Or will you be keeping pullets back as layers/breeders? If so are you looking at pure breeds, crosses or hybrids?
They all have there own place but it depends what your looking for in a table bird, also time to table, weights and the amount of meat against just frame will determine what is your best option is.
If you've read this thread what's your thinking?