How do you get the 2 mercenary groups to help Dany when invading?

So you are supposed to get help from the second sons and the other mercenary group? When I attack Astapor they say they will join me in my wars but they don't spawn. I try to hire them but it seems like they are hired by someone else like Joffrey. Is it just supposed to be a reduced price to hire them?

Side question what education do you usually train Dany in? I guess Martial would be useful ingame since you ride a dragon so you want to max out your combat ability although a diplomat education might be more realistic as most of the time she has others fighting for her. One year of martial training isn't going to make her into a great warrior. Seems like most west commanders start as squirs young then become a knight. Or train at a young age if they are noble born.