I have experiences of mining bitcoin, litecoin, pts, and other ALTcoins before for 1+ year. In my understanding, there is almost no one really love ALTcoins, so
the only reason that people would choose mining ALTcoins is that it has higher profitability (using same hardwares) than minging bitcoin. The profitability of bitcoins and ALTcoins can be found here:
http://www.coinwarz.com/cryptocurrencyAccording to this page, you can see only fastcoin, fireflycoin, and HAMRadioCoin have higher profitability than bitcoin.So these ALTcoins and bitcoin are those coins that your pool should really support, not those lowly profitable coins.
What's even better is that your pool could enable
auto-switch to mine the most profitable coin of same algorithm. In that way miners only have to choose the algorithm (SHA256, scrypt, X11, etc), and get paid in bitUSD/BTS. If so I think this could really attract a lot of miners into your pool.
Thanks