My wifes 43cm Ruby has holes for two cages but the seat tube cage is all but useless. A short bottle is a tight squeeze even with side entry cages. I usually just carry an extra bottle for her. I can carry 5 bottles on my bike.
There are several types of behind the seat holders and some work great and some are bottle ejectors. I've only used an old Minoura that worked well enough. Read reviews and ask in the tri forums.