When it comes to having allies of detachments with your army I.E: Taking a Blood Angels Devastator squad with your deathwatch army. Does that squad/detachment subject to the Kill Team/Mission tactic rules? Can i say put a deathwatch vet squad with the devastators and have the whole thing gain a kill team rule or just have the devastators with a mission tactic?
Nope, each Detachment in an army uses the rules for its own faction. So no Blood Angels with Deathwatch Mission Tactics.
Another question i've had is to what extent can SM stuff be converted to DW. I get that currently the only vehicles allowed are the ones in the rule book but does this also apply for units and/or the weapons? So could i convert say a scout squad to a DW scout, DW Techmarine, DW Command Squad etc. Or are all these things just outright SM and DW can't utilise anything from the SM codex?
The latter, DW can't utilise anything from the SM codex (though they CAN bring an allied detachment/formation of SM, to do it FOR them). Note that a DW scout isn't ever going to happen for obvious reasons.
Can any of the DW in reserves come in on turn 1?. If i have a corvus Blackstar can he come in on turn 1 anyway or some squads with the Sudden Onslaught rule come in turn 1? etc.
Not normally, though things like a Skyshield Landing Pad can give you some wiggle room here and there.
If i take 3-5 DW Librarians could i turn that into a Librarius Conclave?
No, as that's not a DW Formation. However, since it IS a Formation it's a detachment, and as both factions are Armies of the Imperium, you can thus bring a Librarius Conclave in as a detachment of (Battle Brother) SM allies.
If i had a kill team that included a terminator that gave the squad fearless, and the terminator dies does the whole squad lose fearless And/or Its kill team rule EG: Aquila needs 1 Vet squad and 1 or more choices from (Librarian, Terminator, Vanguard, Biker) So if the one terminator in that squad, making it an Aquila Team dies would they lost the ability to re-roll wounds and armor pens of 1 and the fearless/relentless rule?.
Okay, that's effectively two questions. Fearless is an USR. Like some other USRs, it will apply to a complete unit if there's a single model with the special rule in it. However, if that model is lost, the unit also doesn't have the USR anymore.
The Aquila kill team is a formation and you don't lose the formation's rules just because you lose part of it.