Okay so long story short I am an IGCSE student, my name in Cambridge and Pearson is registered as Ali Maged Ahmed Samy Mohamed so the surname is Mohamed, I registered for the ACT as Ali M Samy, will the admissions officer be able to verify that both are the same person? And should I register for the common application with the surname as Samy, Mohamed or is it not going to matter? Thank you!
I think the better solution to that would be to rectify your name with the ACT company. I had a similar situation because I have a really loooong last name / surname (four last names to be exact) but I requested a change by following the instructions given by the ACT website for one of the FAQs: #:-S
“If you need to change either of the names (first and/or last) previously provided to ACT, contact ACT Student Services at 319.337.1270 or write to ACT-Reg@act.org. You may need to provide written documentation to support the change—such as a court order, adoption papers, or marriage license—before it can be made.”
Link if you want it:
http://www.act.org/content/act/en/products-and-services/the-act/help.html
Your college app and you ACT score reports are official documents so it is advisable to provide your personal information in accordance with your other personal official documents (aka birth certificate, passport, etc)
I hope this helps 
How long did it take for them to change your last name?
Since you’re eventually going to need to apply for a student visa, ideally everything should match what’s on your passport.
And do I have the option to choose the last name on my passport?
That is a question best asked of the passport issuing authority of the government of your nationality.