First of all welcome and good luck in starting the program. Second, you're correct in assuming that lat pull-downs are not equal to pull-ups. Pull-ups involve more muscles and core stabilization that just isn't there with the lat pulldown. As such, I would say that subbing bodyrows would be your best bet for substitutions. But, if you don't have a place to do those, then I don't think subbing lat pull-downs for a while would be a waste of time. You work with what you've got. As far as ratio, do you have anywhere you can try out bodyrows? It wouldn't have to be permanent, just a place where you can get a feel for how difficult it is for you to do say 5 body rows, then guesstimate how many lat pull-downs feels the same effort-wise.
And finally, I incorporate other exercises in my simplefit exercises when I have time but I've found a complete lack of need for any arm exercises for biceps or triceps. Varying my push-up stance works my triceps and I throw in chin-ups instead of pull-ups every now and then to target the biceps and my arms are doing fine. I couldn't care less if I ever did another curl.
If you want to incorporate other exercises as your progress try the compound movements like deadlifts, thrusters, bent over rows, presses etc. Focus on those big muscles and you'll force the smaller muscles to grow as well.