Considering that you only have a small tank and no money to upgrade, I would have thought you'd be grateful as long as they do not breed. Remember they can have 40 fry or more in one batch and go on having more once a month for nearly a year even after you have separated the males and females. If you do not separate them, they will just go on breeding. And the fry will be sexually active long before they are of a sellable age, so if you are not able to separate male and female fry, your babies will indeed multiply.
Having said that, it is a good idea to want to vary their diet, to keep them healthy and happy. I think I have suggested this in another post- veggies are a good and cheap source of food for guppies. Maybe two or three times a week instead of flakes. And your bristlenose- what are you feeding him? Again, veggies are good. Peas, broccoli, spinach, sprouts, cucumber- whatever you're having yourselves really, as long as it is not cooked with fat or seasonings, and preferably not overboiled. Just remove the shell on the peas and crush them lightly.