What are the ages and sizes of your older boys and what seats are they using? If one of them is forward facing and close to outgrowing his current seat, one option would be to use his convertible rear-facing for your 9MO and get a nice forward facing combination seat (harnessed seat that can later be converted to booster when the harness is outgrown - the Britax Frontier and Graco Nautilus are the ones that are most frequently rec'd by Certified Child Passenger Safety Techs b/c their high harnessed height and weight limits mean that they'll last until the child is mature enough for a booster - and the Nautilus and Frontier both position the seat belt well as boosters, while most other combination seats are outgrown by height or weight before the child is ready to use the seat as a booster, and most of those seats don't make very good boosters either).
If you need a narrower option, the Sunshine Kids Radian is an excellent convertible seat. Although it's not considered 'compact', it has an 8 year lifespan and high harnessed height and weight limits, and it sits lower to the vehicle seat than the Britax seats do (so it often fits in well in 3-across setups next to a seat like a Britax). Also since it's not on a base, it looks a bit more "big kid-ish" so older kids often prefer it to seats that sit up on a base.
And please do consider keeping your baby rear-facing past the minimum of 1 year and 20 lbs - rear-facing provides significantly better protection to the neck and spine, particularly for young babies and toddlers - please check this link for information from the Certified child passenger safety techs at the carseatblog: http://carseatblog.com/?p=5168