Alternative to BAL SBR

[DIY_Johnny]

Hi chaps 3 questions if I may

1) following on from the Hooley's thread below, I see that priming ply is advised. i have run out of BAL SBR. I have BAL APD which I was going to use to prime the walls (aquapanel / plasterboard plaster) can I use this on the floor too

No one seems to stock SBR apart from Topps tiles is is a pain to get too, is there an alternative to SBR from Mapei or Dunlop etc

2) Also for cermaic floor tiles I have a 6mm notched trowel, would this be ok as a bed

3) Tile spacers, any guide on size or is it just up to me