Question

Do you have to beat Red before you can get a Hoenn starter in Soul-Silver?

Answer 1

There are 3 ways to get one.
1.) After getting all 16 badges, Professor Oak will give you a pass to Mt. Silver where you can fight Red, and if you win, you have an opprotunity to get starters from Kanto/Hoenn.
2.) You can use Pal Park to transfer a Hoenn starter from Ruby/Sapphire/Emerald.
3.) Trade w/ someone who has already defeated Red and received a Hoenn starter.

Answer 2

technically you don't have to defeat red. if you have access to ruby,sapphire,or emerald you can migrate them over to soulsilver.