I have a Squier Affinity 20th Anniversary Strat. I want a floating tremolo but the one fitted won't stay in tune if floating. Is it possible to upgrade to a 2-post system i.e. a kahler and if so would I need a locking nut too? Also will floyd rose posts fit a kahler trem?

You would probably have to rout the body. I woldn't recomend it unless you have a second guitar because routing can be messy.
Hmmm, I see. So it wouldn't really fit without serious modification. Is there any alternative way to get a good floating tremolo fitted?