I have not kept them together and I don't think I would. You say the fish you are thinking of combining are small at present but in my experience these fish grow very, very quickly. My synodontis species have reached adult size within 3-6 months of going into my tanks (i.e. grown from 2" fish to 7" fish at a rate of about 1" per month). Even before fully-grown these fish claim territories and drive rival fish out. All you would gain by putting juvenile fish together is a 3-6 month 'truce' period. After that you will very likely have to re-house one of them. Why not simplify your life and just set up two tanks or, if you only have room for one tank, then just choose to keep one or other species. In my experience the algae eater has the more 'frenetic' behaviour and can make even a big tank feel fully occupied just on its own. My synodontis are active throughout day/night but tend to swim around more languidly, keeping in touch with the substrate, bogwood, tank walls. The synodontis are active but more restful to look at. I would choose one or other depending on the other tank occupants and whether I wanted a languid, restful community or a fast-flowing, active community. Alternatively, if your tank is big enough then maybe you could consider setting it up as a species tank and maybe get breeding behaviours in one or other species. What you do is your choice, but I think you and the fish would be happier if these two were not tankmates. All the same it would be interesting to hear from someone who has successfully combined them. Good luck with whatever you decide to do!