It may take them a lil bit to settle into a new tank to. Where i get my guppys from (i have two girls and two boys) they all came from the same tank. I guess its a combo of things as to why they might be hiding...
1: unfamiliar surroundings
2: possibly prep for birthing
3: water quality maybe somewhat different
4: they miss their mates so are lonely.
5: water temp? (minbe is at 26C deg with pH readong of 7.0 which is neutral)
Id say put them in the tank with the boys. I have live plants, rocks and a tunnel in my gupyy and platy communal tank. It took mine a few days to settle in. From what I know also female guppys dont like feeling like theyre on display if the are preg or preparing...do they have the spot in their side? Are they fat? Maybe add plants or hiding spaces for them.
Mine to this day sometimes hide behind the water outlet pipe but they generally swim around the top of middle of the tank. Give them time to settle in
I also feed mine a combo of crushed flakes, pellets and blood worms. Your lfs should have all that food
