IMO,
FMO,
MGA, etc. are all names used by different carriers for different production levels.
IMO does not indicate an agency is part of a consortium. For instance, American General will give the title
IMO when an agency hits several 100,000 in production.
If an agency is not advanced, it would have no impact on whether or not an agent could be advanced. Actually, many carriers do not advance agencies on their override.
If you are being told, prior to contracting, that those carriers do not advance, then that is an agency level decision. Some agencies may not offer advances to new agents due to the roll-up policy of a carrier. A new agent could write some business that lapses quickly. If they do not have enough as-earned coming to pay the debt, they may stop writing that carrier. Then the debt would roll up the hierarchy.
Understand that the agency is guaranteeing your debt (advance) to the carrier. Many may want a little production on the books from an agent before they assume that risk.
Atlantainsguy has a point about as-earned. Personally, as soon as I had expenses covered, I moved some of my contracts to as-earned. That gave me a good blend of up-front money and steady cashflow.