600 x 300 tiles are too big to be using a dispersion adhesive like Whitestar, you need to use cement based adhesive.
Yes, prime the walls with an acrylic primer like Bals APD, but this isn't to seal the wall, it's to just give a good surface to tile onto.
You didn't say what the tiles are made from, ceramic, porcelain or natural stone etc, but a rapid set flexi would be fine for almost any tile type, but you have to remember it's called rapid set for a reason, so it depends on how quick you are at tiling, as the bucket of rapid set will go off quickly.
To give yourself pleanty of time you can use a standard setting cement based adhesive that will be good to use for a few hours once mixed up, compared to 20 to 30 mins for rapidset.
So, it depends on what tile it is, as to wether it needs extra flexi in the addy and grout.
Welcome to the forums
Beaten by the grumps again
I seem to be going through a phase of priming everything for some reason now
Morning grumpy