Definitely put the female in the 10g with the guppies and the male in the 5g himself. A male will harass the female and possiby kill her in a 10g tank.
I would say no plecs in a 10g as I don't know of any that will stay small enough. BN plecs and clowns stay much smaller but still too big IMHO. For bottom dwellers maybe cories (smaller species) or 3 otos perhaps?
For plant recommendations members will need to know you lighting first. Some offhand that do well in low light which I assume you have as most tanks come with a lower lighting to begin with are java moss, java fern, anubais. Other members will better be able to help you with the plants, my plants do well due to luck, not so much knowledge I'm afraid.