I'm using RTX51TNY - TINY version of real time OS. I've already developed a few projects and ... well I like it. It uses only a few system resources, you can use it even on 8051 (if you don't have to many tasks in your project software). I've tested also this option. But when my project has grown, I've replaced 51 with 52 and finally with AT89C55 (20Kb FLASH).
Code overhead is less than 1 Kbyte, max. number of tasks 16, etc. RTX51 FULL is a larger one, but I have no experience using it for I don't have it in my development package.
If you want more information, you can send me an e-mail to address above.