There are two parts to Smarterhash
Smartherhash.com is the mining income cost estimation engine that estimates income for about a dozen x11 coins and about 3 dozen scrypt coins.
the there are the mining pools. We have pools for most of the coisn we estimate income but for but all - pools.smarterhash.com

The pools look like conventional MPOS mining pools but have been modified to accept command from the smarterhash.com website in the sense that when you setup your miners, you point your miners to our two stratum servers: us.smarterhash.com or eu.smarterhash.com (one is your primary and one is your seconday).

On Smarterhash.com each of your miners is assigned a 'fixed' port number. Every miners has a mining port assigned for each of their machines.

You change coins you're mining through the smarterhash.com control paenl, not by getting on your machine and changing the port number like you do for other pools.

to answer your questions directly:
1) we support X11 - in fact I have 2,550 MH/s of X11 mining power myself
2) Is your pool compatible with nicehash hashrate buyer? - NO, bit it is compatible for miningrigrentals.com
3) Do you have auto-conversion to choosen coin such as BTC, LTC, DASH...? - no your are piad out int he coin your mine.
4) In the world of POW, hashpower is increasing all the time. It means that my subscription, let say 250 mhs, can be a lot less meaningful in future even within 1 year. Will you adjust it to protect subscriber? There is no way to protect the miner - this is the Wild West - you have to protect yourself by buying more mining power all the time if you want more or revenue (or just keep what you're getting today) and even that is never guaranteed because you are always reliant on the price of BTC.