I have a 6600 and do a lot of FMQing on it. I like a size 90 topstitch needle. I usually use Aurifil or connecting threads cotton thread. I have wanted to try the trilobal poly for a while. Need to go find some!!
I also bought the special bobin case. Not sure that it really helped all that much. I also have the slippery mat (sew slip? I bought on Amazon).
It looks like you have a lot of good suggestions above. Did you find anything that helped?