The sand is fine...be careful about the guppies though...you could wake up to literally hundreds of baby guppies in your tank. I'm hoping your guppies are either all male or all female for your sake..you said you want more fish, but you didn't say how many guppies you already have. This could factor out a lot of fish that would potentially need a certain amount of the tank as their territory. I'll list a few fish that won't have any problems with them. Cardinal or neon tetras, cardinals are way cooler in my op though. Corydoras, as they'll stay towards the bottom and won't compete for the food you give to other fish. Harlequin rasboras. Cherry barbs. I definetly wouldn't try tiger barbs with your guppies though.. Another cool bottom feeder are zebra loaches. Or you could add a pair of German, electric, or Bolivian rams to the 30g... Too bad they don't live that long though. Black kuhli loaches are pretty neat but they hide a lot... I don't know I'm kind of rambling off haha, it's really down to what type of fish appeals to you. Like small fast fish, slow moving bottom feeders? I don't know..