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Are the transports for units from a company (Razorbacks, LRs and Rhinos) assigned to the company and do they bear company markings?
Yes and yes. Except Land Raiders. Most of those are part of the armory (the Chapter Master's personal one will often be assigned to him on a regular basis, for example, and I believe some Captains do have personal ones as well).
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What company do Dreds belong to?
Whatever company they're assigned to - I suspect the companies they were part of in life, but don't quote me on that. Various companies have varying numbers of dreads.
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What company do bikes belong to? (I recall reading somewhere that they are from the 8th assault company?)
Bikes are an alternate deployment method for assault marines - so Battle Companies and the 8th Company can deploy as bikes. Additionally, one of the Tactical Reserve Companies typically fights as Bikes in a Codex Chapter (the other fights as Land Speeders).
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Tactical units that are assigned as support to other companies, do they come from the Reserves or are they considered to be part of the company?
They'd be Reserves (or part of another Battle Company).
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Are Tanks (Preds, spare LRs, Vindicators, Hunters) in their own company? (ie 11th or 12th?)
They're part of the armory, and are assigned on an as-needed basis.