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I'm just wondering how units that don't take up a slot in the Force Organisation Chart are treated when it comes to Formations. Two examples are the Tyranid Hive Vanguard Formation and Tyrant Guard units. There's no room for Tyrant Guards on the Formation datasheet, but there is a Hive Tyrant and according to the Codex I can give it up to three Tyrant Guards. Likewise an Eldar Aspect Host Formation can take Wave Serpents as Dedicated Transports. This seems a little more clear cut, but in the case of the Aspect Host units within the Formation receive a special rule that gives +1 to either WS or BS. I've heard that extends to any Wave Serpents, but it seems just as likely that since it's not technically on the chart it wouldn't receive the bonus.
I've been looking all over the Eldar Craftworld's Codex and core rulebook and can't find anything to clarify. Can anyone provide me with book names/page numbers that will clear this up?

Check 'Force Organisation Charts and Slots' in the Choosing Your Army section of the rulebook. There's a paragraph about army list entries/units that do not use force organisation slots (as well as one for dedicated transports, which fall into the same category of not taking up force organisation slots).

Formations are a special type of detachments that tell you exactly which unit entries comprise the formation. Some unit entries allow access to other unit entries; those other entries are allowed in a formation unless there is a restriction against it in the formation restrictions(as long as the "parent unit" is specified).

The baseline rules for detatchments in the brb are in reference to foc detachments.

So hiveguard cannot go into the nid formation that they are not a part of. Dedicated transports, or all the various elements of a Guard infantry platoon are allowed.