@trtj
From what I've heard, there probably isn't a set number that each FTO can send to Waterside. Plus, I've heard that progressing to this stage is really based on each candidates performance against a standard. So, if this is all true, it makes sense that some have been invited already while others still wait for a stage two assessment.
As with everything on here though, this is just based on rumour.
@junio
I doubt you'll find many tips on here to b honest! To be fair, nobody who's been is going to put themselves at a disadvantage. Good luck at your assessment either way!