uC/OS - free until you create a product - has a bunch of "add-ons". Excellent books on how it works. Highly Recommended.

FreeRTOS. Not sure about the security

I am assuming free/low cost for now.

Try ThreadX. Works well on ARM7 TDMI. Quite some time on the market and quite mature.

Hi you could try Nucleus OS

Depending on what functionality the RTOS requires to do ThreadX may be a good alternative to Nucleus - it's basically a downstripped version of NucleusPlus, done by the same people who did Nucleus and left the company to do a new product.

Free RTOS is giving support for ARM7 TDMI from long back and I feel it is one of the earliest OS which i suitable for this. It has far better features compared to other low end OS. Also it has both uip and lwip integrated you can choose which ever is most suitable for your product.

Best Wishes.

BV Ramesh.