Well actually there ARE limits. A ten man squad can't be transported by something with six transport slots. Belive me, I'd love to put ten assualt terminators in a Land Rand Crusader or take ten, break them into two Combat Squads of 5 each, and put some in the Land Raider and Deep Strike the rest. That isn't how it works.
The rules are clear on page 51 of the Space Marine Codex. Rules are also clear about how many models fit into specific transports.
There is nothing vague, unless the argument is applied to Bike Squads. SM Bike Squads go to 8 models, whereas Scout Bike Squads can have ten. According to the paragraph with the description of Combat Squads, ten-man squads break down into two five-man squads. An interesting trick when you only have 8 Bikers. An interpretation would lead you to think two squads of four, but again, GW can't seem to find one competent person to write a clear ruleset.
Combat Squads can only be split into AT deployment and army lists are made PRIOR to deployment. It's clear if you want to read it, not if you choose to interpret based off of - well I don't know what.
If you really want to open up a can of worms, why not ask GW if the Codices take precedence over the main 40K rulebook? I guarantee you they will tell you yes, at which point ask them why Terminators don't get a 2+ AND 5+ invulnerable save. That is after all, what the codex says. But we all know that's not how they are played.
That being said, play what your local shop or game club allows. But please don't mis-interpret clearly written rules as gospel and continue to spread the mis-information.
