Doubting Thomas

I'm trying very hard to control Daisy's introduction to the three cats. They are staying upstairs behind the stairgate, occasionally creeping down to gawp at this new intruder. Doris is very brave and wanders in and out, Thomas and Nibbles are much more cautious and here is Thomas who has just plucked up the courage to come into the sitting room. He is sitting on the back of the settee opposite the one me and Daisy are snuggled on. He is watching her closely. I have had to be very firm a couple of times with her today to stop her chasing Doris. She is quickly learning a firm deep "No!" means "don't you dare chase that cat!".
I hope it won't be too long before they all get along.