yes I'm definately one of the 'thinkfish is rubbish' school of thought, i've seen some terrible stocking suggestions from that site in the past.
the general 'rule of thumb' that i work to is in the first 6 months of the tanks life you keep to around 1" of fish per gallon, then after that you can increase it to 1.5 or even 2" per gallon providing you have edxcellent filtration and keep on top of the maintenance.
also you should take into account the bio load of the fish, danio's are low waste producers so while they're actually a bit over, i just count them as 1" of fish each whereas something like a platy which is a similar length fish actually produces much more bio load on the tank and should be counted as 2/3".
however this just gives a rough idea of how many fish you can fit in, far more important is to ascertain weather the fish you have are actually suitable for the tank, and the danio's unfortunately are not. While they are just small fish they are incredibly active and do need a decent volume of water to swim around in, for a group of 10 of them I'd recommend nothing smaller than a 30 gallon tank.
So if you want to keep them then I'd recommend you upgrade your tank or just get a second one, alternatively you should take them back to the fish shop. Don't expect your money back on them but they may give you a few quid of store credit.
For me I would get the 30 gal tank, I'd then count your stocking as this
10 x danios = 10"
4 x corys = 8"
3 x male guppy = 4.5"
Total - 22.5"
So that then means you've room to increase your stocking by some way, as the tank would be immature still I'd just build up to the 30" at first, I'd recommend 2 more cories as they really do like being in a decent sized group, then you could go with something like the honey dwarves as suggested above giving you this
10 x danios = 10"
6 x corys = 12"
3 x male guppy = 4.5"
2 x honey dwarf gouramis = 6"
Total - 32.5"
And then after a few months you could add a school of tetras, rasboras or something like that comfortably and maybe a bristlenose plec or a couple of siamese flying foxes something like that.
