Presumably your Alliant login is for all of your accounts, right? After you provide the login, they’ll ask for a security word (one of five I think that you’ve set for Alliant), same as would happen when you login from a new device. After that it will ask you which Alliant account you want to link. The question is weird though, it asks “which one is your primary account”, but what they really want to know is which account you want to link for ACH.
It’s not required to push from another bank, it’s only required to push/pull from SoFi.
Alliant has a daily ACH limit of $25K. SoFi has a daily limit of $50K but a monthly of $100K. If you’ve got a bunch of money to move, it’ll take a while. Writing a check might be better, but then you’d lose some interest, and mobile check deposit also has limits. Wire transfer is another option, but it’s not free and may also have transaction limits, though they should be higher.