With the guppies, your tank is starting to get full. If you have a common plec, these things grow huge, but that is another story.
Most barbs are shoaling fish, so you need to keep a lot of them. Tigers, with anything long finned, guppies for example, is just not worth it. In your tank Cherry Barbs might work. They are a non shoaling fish best kept as a pair or trio, 1 male 2 females, (2 males might fight). Small colourful and harmless.