For the most part you should not have any trouble with the leg press/sled since the back is supported/braced. The rounding of the back should never be a issue. Can gain respectable size with either, or both, in a leg workout. Of course there are always exceptions to an injury happening to the lower back no matter how cautious you are. When going back to regular squating, take it easy at first. Leg presses, a sled or any thing else is never a complete replacement for heavy squatting.
Might try some Romanian DL's to strengthen the lower back. Beginning light at first and with-in a higher rep scheme. 12 to 15 reps. Also hits the hams very well. As you get use the exercise, and stronger, some respectable weight can be built up to. Consider GoodMorning also. Hitting the abs hard is another method of insuring a stronger lower back. Back and abs work together. If one is weaker than the othe than you may be prone to injury. Good Luck.