ok... before you go messing things up for yerself, I'll tell ya what you should do...
veg all yer plants together - won't do no harm.
you should be able to tell the sex, possibly, by the time you start flowering.
http://www.marijuanapassion.com/forum/showthread.php?t=35407
when you start flowering, start inspecting daily, and after a week or so, your males should start to "drop their junk" (producing pollen sacks) - don't worry... they will just start growing their pollen sacks... pollen actually being released is still a ways away...
take the males and isolate them away from the flowering room - if u have a box or something in a room, put the male in a different room... on another floor, if possible - just somewhere with little or no air movement... a window sill in a corner works good... just be aware of heater registers if u have fan forced heating.
Just let the male finish blooming at it's own light source separate from the grow and collect the pollen as he drops his load... I, myself, would tie down branches with pollen sacks on them over top of tin foil to collect the pollen when the sacks open up. They start out green, then go yellow as they mature, and open up.
Every time you've been around yer little dude when he's been having a good time, you have to wash up and change your clothes if you plan on going in yer flower area again.
When you've selected yer female to pollinate, take it out of the flower area. use a
Dry Q-tip or paintbrush dipped in the pollen to brush over the pistils (little white hairs).
wait a good 30 mins, then lightly mist the plant to kill any stray pollen that might contaminate the flower area. - the water will kill excess pollen, this is why u use a dry implement to pollinate with ... then go wash up again yerself, cuz yer going back in the flower area, and you prob got pollen on you and/or your clothes yerself...
put 'er back in the flower area and yer good 'ta go
simple steps to follow, but u should do them them way I told you, or yer gunna be :hairpull:
just like having sex in real life... if yer not careful/protect yerself... you could get yerself into trouble with unwanted pregnancies (pollinations)....