What size is your tank? How many guppies do you have already in there? How long has it been set up/cycled?
As with any cichlids, they do have certain requirements in order to thrive in the aquarium. For example, German Blue Rams are fairly small cichlids but they still exhibit all the usual nesting behaviour (in mated pairs) and will chase other fish out of their "territory" to protect their eggs/fry. They also need really good water quality, so regular water changes would be a must.
I would say always do some research into the needs of a particular species of fish before rushing out to buy them, just to be sure you can give them a good quality of life.
Regards, Athena