The one inch of fish per one gallon rule is a just guideline and really hasnt been proved useful. You can overstock your tanks a bit, provided you have good filtration and keep up with the frequent water changes.
If Im correct, 10 UK gallons is 12 US gallons. 6 WCMM's, 1 Sparkling gourami, 2 scwartz's corys wont be too much of a bio load for a 12 galloner so I say go for it. But one problem, cory's like to be kept in groups of at least 6. 6 clouds, 1 sparkler, and 6 cory's is too much of a strecth for a 12 gallon tank. To replace the cory's. you could try pygmy cory's which only grow to about and inch and a half.