Hi there,
Taxi is used only inside the cities ... but you can use the Louage ... it goes from a city to another ...
So simply you can take the louage from Monastir to Tunis and once you are in Tunis you can get a Taxi to Bardo ( by the way it's a wonderfull musuem ,,,, you gonna love it ... you gonna see how reach Tunisia is )
Sidi Bou Said ,,, once you are in Tunis ... you just need to get the TGM ( Tunis / Mars ? Goulette ) and it'll bring you to Birsa ( Carthage Birsa )
But if i was you ... i would just rent a small car and i'll drive myself ... that's cheaper and easier .. and funnier
Cheers
Makrem