Do i have to enable XCache data store in config.php ?
Do i need to set the xcache admin pass in config.php?
What php handler i should use?

you ned to make sure your your webhost allows it, or supports it

I;ve missed some note the dev wrote, the xcache must be enabled in config.php but the other 2 questions still need answer about best php handler for xcache because i'm getting internal server error with some of them, i'm using suphp atm.

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Cache system can unfortunately not be deleted

Interesting.
I started looking at this in the summer but had to put it on hold.
Might have to play with this a bit.

The datastore cache does not have to be set to xcache. That was only a recommendation.
The xcache admin user and pass must be set in config.php. Read the install instructions to see the code necessary to add to config.php to do this.
As for your question about a php handler, my mod does not care which you use, but xcache might need a specific handler - I'm not sure. You will have to ask google or look on the xcache website about installing xcache.

Do you have any mods installed? If so do you mind listing them?

Also what do you mean "cache system can unfortunately not be deleted."

Not sure which "default settings or suggested settings" your referring to, but here's basics of how to tweak xcache.
Actually there isn't really much to it.
Have a read here at what the different options affect:
http://xcache.lighttpd.net/wiki/XcacheIni

It would be useful for you to have xcache admin. It will tell you your cache sizes (php cache = xcache.size, var cache = xcache.var_size) in megabytes, and will tell you how many slots are available for each (php slots = xcache.slots, var slots = xcache.var_slots).
To see if you need to increase, or could get away with decreasing, those numbers, check how much memory is being used as well as how many slots are being used, then tweak accordingly.
If you are using all memory, you would increase memory.
If you are using all slots, increase the slots.
You want to leave extra room for the cache to grow, but keep in mind any memory you give to xcache takes away from memory available to the rest of the server, so it is up to you...

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I'm asking about if i really need to set the xcache user/pass in config.php because i couldn't authenticate to xcache admin panel until i turned off the admin authentication.
Is it ok?