Yeah sounds like the cherries would be gone pretty quickly with mollies and such.
The fert dosing schedule on a non co2 tank is kind of a tossup. Personally with mine I dose right after water changes, so pretty much twice a week. However, my saving grace has been keeping the light low. I had algae issues when I quit dosing excel regularly and the only solution I found was to raise the light fixture a bit to lower intensity.
If you work with the PAR chart on plantedtank.net it will give you a good starting point to work with, less light = less demand on ferts/co2 and also less growth, both in algae and plants. The upside to this is the algae doesn't go out of control as quickly as it would with higher lighting so you may be able to find your balance and then slowly increase intensity from there.
As far as diatoms go, I just wait them out or let something eat them. You could manually remove it from leaves and decor with a light brushing, but otherwise it'll go away on it's own eventually. I've had diatom problems in tanks with nothing but river rocks, clay, and other non-sand substrates so it's not limited to sand, I think that silicates or something else in tap water plays a role as well.
