My long arm dealer told me not to use titanium needles - but I don't remember why . . . when I was making custom lingerie, I always used the ball point needles - usually a 75. Other than special purpose, I rarely use anything larger than an 80 in my domestic machines.
The old Boye needles came in 30-40, 40-60, 60-80, 80-100. Not sure what those equated to though. You would get an assortment in a tube, with one tube having the first three sizes, and the second tube having the last three sizes. So the two in the middle you got regardless of what tube you bought.
Getting the right needle for the fabric is the easy part - getting the right needle for the machine is the harder part. Once I get the Wheeler Wilson #8, I'll have a machine for each size needle sold in the Boye round needle display case.