I was trying to optimize Tixati and checking the maximum possible downloads for my bandwidth. When I decrease it, the number of downloads still remain to the maximum set previously. How do I send some of them back to queue?

Did you wait long enough? :)
Have you enabled "use free slots from uploads/downloads" in the same window?

I'd not worry about "optimizing" the amount of active torrents unless your speeds for each are under <30 KB/s. As long as you have 0 packet loss and good ping the individual low speed shouldn't matter.