First off this is not a critique my routine thread and I'm not asking for permanent sqaut substitutes. I've picked SS and I'm going to do exactly what it says.
The one thing I don't have is a squat rack. I've looked around my house for anything I could use to make one but couldn't find anything... I'm saving for one but for the time being what can I do? I did my first day yesterday and I just did bench, press and DLs. I workout at home btw.
Secondly is it ok to do light cardio and abs on off days?
It is always ok to do Cardio as long as you're getting enough rest and recovering. Abs CAN be over trained and couple times a week wouldn't hurt since they are already being used to some extent in your Core lifts.
Wendler's 5/3/1: PR's
Overhead Press - 125x8(158)
Deadlift - 320x8(405)
Bench Press - 200x11(273)
Squat - 240x10(320)
So how much ab work on my off days is alright to do? 3 sets of sit ups till failure? I'm unsure on this one.
Also what can I use as my squat "stand-in" exercise while I save up for a squat rack/cage?
I would save your off days for recovery and do your ab work after your compound lifts.
You can clean and press some weight and then bring it down behind your head to squat with. You should be able to easily squat whatever you can clean and press, but you can at least get some form practice this way.
Granted, I've never personally tried this so I'm not sure how difficult it will be to position the bar correctly when bringing it down behind your head. If it is too difficult to position it correctly you could just do overhead squats for the time being.
You could also do Zercher squats for the time being.
Starting Strength is based heavily on the back squat, and it's not light form work either, it's heavy gut-busting effort. Without the squat, it's just not the same routine and really isn't a good beginner program any more.
I would suggest looking at Hardgainer or HCT-12 and using the traditional deadlift and bulgarian split squats as your lower body lifts. At least until you get a squat rack, then I'd definitely start squatting.