Your technical question: the latest version of eloipool has module based authentication. There is no SQL based plugin yet, at least not on all the pools that use eloi that I know of, but it would be appreciated if that is contributed to the code. eloipool is AGPL so you are obliged to do so if you use it on your pool
It can even be written for you if you want, but I understand that you have the technical know-how.
About the payment model. What you are asking is impossible.
No variance = pps
low variance = dgm with the correct settings, but that implies the same drawbacks for the poolops as PPS so that's impossible, OR being a big pool.
you could try to get lower variance by using PPLNS with a very high N, but still people would only get paid when a block is found therefore the variance can be high if several days go by without you finding any blocks.
So basicly, you can't do what you want.
Also note that slush's scoring algorithm is hoppable and should not be used. I believe slush announced he would switch to dgm at some point, but afaik he didn't complete that yet.
About stratum only: I guess that's the best way to start. Getwork will cease to exist. GBT has it's advantages, but also it's disadvantages.... future shall decide how that goes, but for now stratum is just fine.