Ok.
The goal is to achieve a mold and a master casting in a durable material.
As for the casting urethane rubber was used before platinum silicones came out, but now there is the platinum silicone option too. You want a flexible/soft material as the molds you will be using in your final process are USG No 1, rigid plaster. So the rubber should be more likely to be flexible foam filled. To aid easy demolding without damaging the plaster.
Also you want a rubber with negligible shrinkage to preserve dimensions, moreover if your final casting material is latex, that shrinks in a high percentage, and you don't want your final latex castings to reduce even more compared to the original. This suits urethane rubber and platinum silicone.
I don't know if you are familiar with these rubbers. Urethane rubbers need release agents to be casted and molded. Platinum doesn't, except to release from itself. On the other hand Platinum silicone inhibits with different chemicals, and one that really hurts is latex and the sulfur it carries.
I'm saying this because that might tell you what molding materials and process to choose.
Now, what are you after?. Will you be starting from a clay sculpt or are you after making master copies of what you already have?. Depending on that you will have to see if you can go with rigid molds all the way or if you need a silicone flexible mold at some point.
If you start with the sculpt you could mold it in a good rigid and stable material. For example acrylic resin, epoxy+fiberglass...and cast in platinum silicone filled with a good flexible foam and then remold that in USG 1. The hardness/flexibility/stiffness of the rubber and foam will have to do with the shapes and undercuts in the design.
If what you want is remolding what you already have, your latex castings are most likely shrunk so it's not and option for remolding. It would result in smaller castings at the end (could be problematic for masks for example). If this is important in what you are doing. Also your plaster molds are contaminated with latex so you can't cast platinum silicone in them in first instance. You might have to cast urethane rubber in them, sealing and releasing the dang out of it as it's like glue and plaster is porous (it will grab like hell if not sealed and released), and then remold that casting.
Another option is using a silicone mold (platinum better) in first instance, on a sculpt or a rubber casting . Then you would have a durable mold to cast almost anything in it (not latex, hard to do) and remold those castings. Another option is Tin silicone (no release either), but a small contraction rate. Maybe not important in what you are doing, it depends.
This is the general process but there could be many workarounds and options, so I think the first thing is to know what exactly you are working on (masks, props...) and what the designs and necessities are. Also, what are the gaps, and the techniques you are not familiar with so we can help. I understand you do make molds with plaster, so that's a good start.
