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I'm interested in starting a Mantis Warriors army due to Divination Librarians w/o allies and the fluff.

I looked at the chapter tactics and it says models get MtC and HoW, as well as Furious Charge if assaulting from terrain unless a model in a unit has a variant of the bulky rule.

If I attach a Librarian who isn't bulky (so power armour or equivalent) to a unit of Centurions, would the Librarian lose the Chapter Tactics rules, and if he left, regain them?

What I mean by this is that the Librarian is his own unit and would get the rules for Pro side, Con side would be the unit would a mixed unit w/ a bulky model in and wouldn't get CT

 Frozocrone wrote:
...
I looked at the chapter tactics and it says models get MtC and HoW, as well as Furious Charge if assaulting from terrain unless a model in a unit has a variant of the bulky rule.

If I attach a Librarian who isn't bulky (so power armour or equivalent) to a unit of Centurions, would the Librarian lose the Chapter Tactics rules, and if he left, regain them?

If I'm understanding correctly, you've got the answer there.
When the Libby joins them, he has joined that unit. At least one member of that unit has the Bulky, Very Bulky, or Extremely Bulky rule, so he would not benefit from MtC, HoW, or Furious Charge.
When he is on his own he would gain the benefits again.

 Frozocrone wrote:

What I mean by this is that the Librarian is his own unit and would get the rules for Pro side, Con side would be the unit would a mixed unit w/ a bulky model in and wouldn't get CT

I don't understand what this is supposed to say, sorry.

 Frozocrone wrote:
If I attach a Librarian who isn't bulky (so power armour or equivalent) to a unit of Centurions, would the Librarian lose the Chapter Tactics rules, and if he left, regain them?

He wouldn't lose the Chapter Tactics rule, but would lose any benefit from the Shadow Killers half of the rule (but keep Children of Prophecy)

 Frozocrone wrote:
What I mean by this is that the Librarian is his own unit and would get the rules for Pro side, Con side would be the unit would a mixed unit w/ a bulky model in and wouldn't get CT

Once joined to a unit (say, Centurions), the Librarian is not "his own unit" any more, he is part of the unit he joined, losing the benefit of Shadow Killers until he leaves the unit again.