Baggage rules when travelling on multiple airlines can be difficult, and can vary based on the country that your journey starts in, and specific flights involved with the flight. What's more, the rules can be different in one direction than in another.
Singapore Airlines has a page on their website describing their different fare types, and this page explicitly states that
Do note that the conditions that apply to your fare type are only
applicable on flights operated by Singapore Airlines. If you are
travelling with a partner airline, their conditions of carriage will
apply.
You mentioned "El Al" and "first leg" together, so I'm presuming that you are starting out with them at least for one of your directions of travel. Presuming this flight doesn't originate in the USA, then the "IATA Resolution 302" rules would likely apply. These rules are a little complex, but Air New Zealand has a somewhat simplified version of them here that you can look at to determine who's baggage rules would apply for this flight.
If El Al is the "Most Significant Carrier" for your flight, then the Singapore Airline baggage rules will most likely not apply, and you will be charged under the El Al rules. If Singapore Airlines is the Most Significant Carrier then your 30kg limit should apply.
