Answer is no, male and females should not be housed together, there would be too much stress involved. Chances are that the female sorority would bully the male.
Males shouldn't be kept together, no matter the tank size, again if they are not fighting, it will cause stress. Betta Splendens are hybrids and differ from wild bettas in the fact that splendens were bred to fight, hense the males being extremely handsome and colorful.
I would say a min if 5 females together, I have a 60ltr tank with 8 females in, the 10ltr 'rule' doesn't really apply to females in a sorority. The more there are, the less likely they are to squabble but I wouldn't use anything less than a 30 ltr for a sorority.
No matter the species of splendens, they will jump at somepoint, so it is not advised to have no lid, although, you could try to use a form of netting, instead.
Some people have had no issues with guppies and bettas, others have had it. It is generally advise not to keep both species together but if you plan well and have lots of hiding places, then it may well work out. IMO use real plants and not fake ones, better for all the fish in the long run.
I would read up on bettas esp sororities and research before purchasing. When choosing your females, watch them carefully, pick the lesser aggressive ones and your tank should be ok.