Welcome aboard! Those add ons sound like a really good idea, they will add more bulk to your armies and variety. The veterans set is my favourite roman box.
Like you I have built both sides, British Celts and Romans and my Britons (when all painted) will out number my Romans by about a 2/3 ratio, with 3 Britons for every 2 Romans, but some of my Romans are Auxillaries (Warlord do a set but they are currently out of the webstore for a little while, being repackaged I think) who are less value than the standard legionary. To be honest my armies were it for WAB rules not Hail Ceasar, but I would think that if you are talking legionaries vs celts a 2:1 ratio is fine.
You might also want to think at sime point about buying some command figures for each side to represent the generals. I did my Celtic chief in a chariot.
"I've been a frickin' evil doctor for 30 frickin' years! So cut me some frickin' slack."