Help me understand this since I am not knowledgable on this subject:

So, the input voices and instruments, if they are loud enough can overpower the 200 w Logitech. Can't we adjust the mixer settings to prevent this ? The Behringer mixer is NOT a powered mixer. So, I am a bit perplexed at the moment.

UD: you're saying that if I connect the output (mono?) of the mixer to the "line in" (same line as mic line) and flip the switch to "IN" position on my PSR2000, I can make this work ? Use the compressor setting on the mic setting menu buttons ??
I thought 200 watts is loud enough but I guess what you're saying is if I use up 50 watts on the kybd, 50 watts x4 singers for the voice = 250 watts total , I will run out of headroom. Right ?

Starkeeper/Gary: I guess I just have to reduce the number of invited guests. I did have 70 people before and they were spread out to the family room nearby and even to the backyard. We probably have to do major rearrangements to accomodate these number of people. If it continues like this for several times then I will just have to rent a community center nearby or workout a deal with a nearby church. For now, they have to fit in somehow.