moving more than 8 sims together

The game has a limit of 8 sims so no, you cannot do this. There are mods that will allow you to have more than 8 sims in a household. however that doesn't mean you will be able to merge your two families as the game will refuse. You can with a mod have more than 8 (children, grandchildren) however be aware that if you load the household in CAS the family will be truncated to 8.

(btw I am not working with origin but Ive been through this before so ik whats going on) Unfortunately sims households can only have 8 sims at a time, which might be why you cant merge the households together. The only possible way to do this is to download a mod that could allow you to, but viruses could be spread through mods so I would'nt try to if I were you. Hope my answer helped. - A nerd making a difference

Thanks, and yeah, that's where my research lead me to... I was wondering tho, is it possible to replace a customer sims with one from the Gallery ? (Like using the cheat "sims.spawn" (or something like that I can't remember), then modify the family (through CAS or simple edit mode) and replace this sim with one I have in my gallery? 

Yeah I saw that too... but like I replied to the latest reply (I didn't know if you would see and I wanted to know if you happened to know if this was possible...:

" I was wondering tho, is it possible to replace a customer sims with one from the Gallery ? (Like using the cheat "sims.spawn" (or something like that I can't remember), then modify the family (through CAS or simple edit mode) and replace this sim with one I have in my gallery?"

Yes you can replace one Sim in the current household with one you have in the gallery using a cheat .

Just in  case you make a mistake I would recommend that you do a manual save first.

Press ctrl+shift+c to open the console.

type testingcheats on (enter)

type cas.fulleditmode (enter)

close the console.

Load the family in CAS

Hover the mouse over the thumbnail of the sim you want to delete and click the X that appears.

Open the Gallery and select the sim you want added and press the Place household button.

Do any other changes you may want then save and continue.

Sadly, you need a mod to increase a household size beyond 8.  MC Command Center is also needed to allow for more than 8 household members (this number includes pets).