In all honesty?
Telling ex before as a courtesy, particularly as he offered you that courtesy would've been the right thing to do, not only in terms of your co-parenting relationship with your ex but because essentially leaving it to a 7 year old is unfair to your daughter.
Also I personally think 5 months is too soon. 6 months absolute minimum, 1 year better - as you don't know until the Rose tinted specs effect (various biochemical a early in a relationship + psychological factors) if the relationship is likely to last and that takes at least a year. The fact you're still referring to him as 'new' boyfriend also suggests you're still very much in early relationship mode.
How long was he dating the gf he introduced dd to? Wow! 2 weeks is WAY out of order!
"and ‘need to know’ stuff." This IS need to know, you're introducing HIS dd (not just yours) to someone who may play a significant role in her life, he has a right to be given a heads up on that, not least so he can properly deal with any issues that may arise either with you or dd.
Non payment of maintenance is a separate issue, but I appreciate it is frustrating (been there). Have you tried through cms? Ah self employed (they really need to tighten that loophole) in some cases you can request that they check with income tax dept re declared income, possibly that they investigate that his lifestyle is commensurate with his supposed income.