The 99k is a low shank machine so the 306 feet should work but the 401A (slant shank) feet won't.
I got a 99k a couple months ago...in a cabinet (still needs to be refinished) for only $20! I REALLY didn't need any more machines but, for that price, I couldn't pass it up!
Anyway, I absolutely LOVE it! It sews beautifully & it's just so darn cute. I hope you're as happy with yours!