While a sub domain is treated as a "separate" site, is there any weight is transferred from the main site. I have something that I want to add as a subdomain as it makes far more sense that way, but there is a need that the section I wish to add be indexed fairly fast. Therefore was thinking of maybe adding it as a sub page of the main site instead? Then release the subdomain version and delete sub-page once this short term need has passed?

I always heard that Google just loves sub-domains. I don't know if it's true (or to what extent) but I believe you better go with a definitive URL from the start.

Moving things around is not good for the indexing.

I agree that moving things around is not encouraged after the indexing. I believe that putting it up as a sub page / domain has the same effect. What matter most is your inbound links and content.