Drop the peice of cucumba in boiling water for a few minutes, get a plant weight thing, and attach the cucumba to it with a rubber band, then let it sink to the bottom of the tank. Yes to many bloodworms can cause constipation as they are high in protein.
The guppies will, you should have that tetra in no less than a group of six, as it will be stressed on its own as it's a shoaling fish, what is a flagtail is it a plec.