we tried ethereum by instruction http://ethpool.org/faq, but it seems impossible on our gtx 750 - we get either "not enough memory" or very low hashrate.
Ethereum is well known to not work well on the 750Ti, it doesn't have enough RAM or some such issue. The GTX 950 is about the lowest Maxwell card that works well with it (I get appx. 10Mh out of each of my GTX950s) but Ethereum is a coin that prefers AMD hardware.
X11-based coins might be profitable on your machines, like DASH, but your machines really aren't optimised for mining (too much CPU that soaks a fair bit of power, very little GPU horsepower).
Is there a reason other than cost that you are running the lowest-end of the Maxwell cards?
