Hi karia,
Welcome to our Mattress Forum! 
The good news is that you can always s make a mattress that feels too firm a little softer by adding a topper. The choice of material is good too as both Talalay and Dunlop latex have great pressure-relieving qualities but also provide additional secondary support for the recessed areas of your body. Latex has an unusual combination of surface softness and deeper firmness/support that comes from its elasticity, and can certainly be an effective way to add some additional “cushioning”. Toppers are designed to add comfort to a mattress that does not sag but is too firm and needs some pressure relief qualities. When you use a topper over a mattress the topper will compress and in combination with the layers below it will take on the shape of your body profile. Softer toppers will compress more than firmer toppers. This “cradle” formed by the upper layers re-distributes weight away from the pressure points of the body.
Sometimes choosing a topper can be as difficult as choosing a mattress as it takes a bit of trial and error to dial in and find the exact combo of support/comfort best for your needs. If you need just a bit of “cushioning”, then 1" of Talalay might be all thickness you’d need, but before you decide on how much thickness and what comfort level do you need (soft, medium soft, medium, firm etc.) you’d take into account a few other factors to select the appropriate thickness/firmness for your needs and preferences and for mitigating your sciatica and hip pain.
You did not mention your primary sleeping position, nor your height, but if you are a side sleeper you may need a bit more thickness for your topper. There is more information about choosing a topper in post #2 here and the topper guidelines it links to, which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market), can help.
In general, a good starting point for a good comfort layer for a side sleeper is 3" and then depending on weight, body shape, preferences, and the firmness of the support layers, to go up or down from there. Most side sleepers will fall in the range of from 2" - 4".
Toppers, like mattresses, are a comfort choice , so your preferences for components, fabrications, thickness, etc. will all factor into your selection. Here are a few important guidelines
~ If the mattress needs just a “touch to a little” extra softness or a softer “surface feel” … then a 1" topper is usually enough.
~ If you need a little to a fair bit of extra softness and pressure relief … then a 2" topper would be in the average range. Given that you are lighter this may prove to be a good option for you.
~ Generally Heavier weights do better with firmer and thicker toppers. Lighter weights generally do best with softer and thinner toppers.
At a quick glance I could find some of [url= https://mattressunderground.com/the-underground/our-trusted-members.html]our trusted members[/url] that offer latex topers, but most of the Talalay toppers that are offered come in 2"-4" thickness. You may consider giving the manufacturers a call to see if they would be willing to slice a topper for you to fit your desired thickness, or enquire if perhaps they would have some other unlisted options that may be of help to you. Just in case you are reconsidering the topper material from Talalay to Dunlop I also listed some manufacturers who offer 1" Dunlop toppers…
DIY Natural Bedding has a latex calculator and it seems that it offers the 1" thickness option for organic Dunlop latex.
My Green Mattress has a 1’ Dunlop topper
DIY Mattress has 2" or 3" NR Talalay & Dunlop toppers
Talalay Toppers | DIY Mattress
Arizona Premium 2" or 3" Talalay & Dunlop toppers
Foam Sweet Foam Has 3" toppers both Talalay and Dunlop
Latex Mattress Factory 2" or 3" topper
Sleep EZ has 2" or 3" NR Talalay & Dunlop toppers
No matter what you end up selecting, I’d make sure to ask the company about the topper’s return policy as some of the retailers/manufacturers do not accept topper returns or exchanges.
Once you get a chance to peruse the information and links mentioned above I’d make sure to check out our Trusted Members here Most have good quality toppers and I have no doubt that they will not hesitate to help you find the best combo and thickness/softness they have available that works for you. As you may have gathered, It usually takes a bit of trial and error and a bit of time spent on the mattress to ensure that you “dialed in” the best mattress/topper combination suitable for you. I’d make sure that whatever you chose has a good exchange policy as you won’t be able to take your new mattress with you to a shop to test the mattress/topper combination as they do not work in isolation.
I hope this helps and let me know if you have more questions and I’ll do my best to assist you.
Phoenix