The issue of any invasion is not how many Clans are involved or where they attack from (though this is contributary to the actual problem), the primary issue was logistics.
The Clans failed at this, utterly, and any invasion through Capellan spoace would more than double their problems. It would be like Great Britain invading Russia via Siberia. The Clans failure was dirven by both thier lack of effective logistics and thier own strictures on all out war. These issues mutually compound with every light year travelled and when adding another 1000LY dogleg to the already long line of supply, you get into real trouble.
Even if every Clan sent thier entire front line force and struck every state at once, they would still fail, as the IS would be immidiately unified and strike back as one. The only real effective invasion would be a 20 Clan race to Terra through the Commonwealth and Combine, using solid frontline forces and a swath or second line troops for garrison work.
Still, that option would turn the IS area taken into a miniture Kerensky Cluster, with no firm borders for each Clan. No matter what happens, Terra really is the Clans unreachable star, due to thier lack of foresight and own cultural strictures.
