welsummer chicks

hi, I have recently brought 4 chicks home, around 11-12 weeks old now.  I got them from a local garden centre, mainly becoz they are pretty, bought for pets.  lady on duty wasn't sure of the breed but I suspect welsummers from their colouring, also 'thought' they were all girls.  my understanding is that this is an easy breed to sex from day old?  I got them at 6 weeks.
question is how do I tell if any are boys (preferably before any crow!).  two of them are much paler feathered than the other two with small pale combs, one of these is tiny, more like a bantam than a full sized hen (was assured they are all the same age and breed), the other two are much larger, darker, with red combs.  am hoping its all part of the variation in colour and size as don't want rid of any of them, but am growing suspicious of the two larger chicks. 

did try but file too big!  i'll try and resize the pics and see if I can get them up.