Guppy Tank Friends

How long is your tank? White clouds are quite fast swimming fish and need at least 60cm swimming room. And they need temps in the 14 - 22 deg C range as they are sub-tropical.
Do you desperately want white clouds or would you consider something else?

Whichever fish you ultimately get, yes it is OK to add half a shoal at a time. They won't be in a too small group for more than a couple of weeks and the first batch will cope for that long.

Do you like ember tetras or any of the Boraras species? Boraras brigittae or B urophthalmoides are quite easy to come by. Though you need to check if they are OK in your water parameters. If you have hard alkaline water, look at celestial pearl danios (aka galaxy rasbora)