Hi Beanie,
The “weight limit” isn’t because of durability but because thinner latex mattresses can sometimes feel too firm for some people. There is more about the effect of thickness in post #14 here but the firmness of the latex will also make a difference. If the mattress is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) then you will be fine regardless of the thickness.
Latex is the most durable of all the foam materials and will last you for a very long time. You can see an example in the video here of a thin latex mattress that was used on a regular basis for almost 50 years (although I wouldn’t treat this as the “norm” for any mattress). The best use for a topper would be if you need some extra softness and pressure relief but if your mattress feels the way you want it to then there would be little point adding a topper.
It’s a good idea to have some ventilation and airflow under any mattress (not just latex). For short term use if you leave your sheets and bedding off during the day and put it on its side to let it air out on a fairly regular basis you will probably be fine but I would probably put something like the bed rug here or a thin slat system such as here underneath it to add some airflow.
Congratulations on your new mattress 
Phoenix