Tokis-Phoenix
^_^
Because your friend is a newbie is probably best to stick to hardy fish which are not going to all imediately die off at the slightest sign of water quality issues etc- i trust you will explain to explain the basics of cycling tanks and maintaining a clean tank and checking the health of your fish etc 
.
Hardy small shoaling fish which can go in a 15-30gal i can think of are;
Danios (zebra, leopard, pearl danio's etc- they're all hardy very active little shoaling fish)
Bloodfin tetras
Black neon tetras
Cherry barbs
Glolight tetras
Penguin tetras
White cloud mountain minnows
Black or White skirt tetra's
With small plecos, i have personally found clown plecos to be very robust little plecos and they grow to a small size however they do need driftwood/bogwood to eat and good filtration in the tank (their poop is like sawdust). Albino and peppered corys have also also been very reliable fish too in my experience.
To add colour and friendliness to the tank, you can't go wrong with common livebearers, however i would personally advise platys over guppys as platys seem to be hardier in my opinion- swordtails are also good though. The downfall with mixed gender groups of common livebearers is that they will produce fry, however there are many ways you can deal with fry. With swordtails, you can only keep one male in the tank unless you have a very large tank and an exceptional amount of females per male (like 15 or more), its best to either have 2-3females or more to a male, or just a trio or more of females and no males. If you opt for platys you can keep more than one male in the tank as long as you have 2-3females per male or have an all-male group of at least 5-7males or more, or you could just have an all-female group of platys etc.
The good thing in particular about fish like platys and swordtails is that they come in almost every colour and patterning imaginable so are great fish if you want friendly inquisitive and colourful fish in the tank- unlike a lot of other fish like danios and tetras, they actually notice you and will come to the front of the tank to great you excitedly etc
.
Guppys are not as hardy as they used to be and tend to be riddled with diseases now days while mollys often end up needing slightly brackish tanks to survive and thrive in the long term, however if your friend opted for a slightly brackish tank then he wouldn't be able to keep any of the tetra's, danio's, minnows or catfish etc i have listed so far as they all don't cope well in salted tanks in the long term.
Gourami's are good for adding colour to tanks but they are not the hardiest of fish and you do need to take into consideration the genders of the gourami's you have as the males can be quite agressive towards each other and other fish- gourami's and fish like female betta's thrive best in heavily planted tanks (preferably with plants that grow all the way to the waters surface or has floating plants etc) with gentle current so would be ideal though if that is the sort of tank your friend is aiming for.
.Hardy small shoaling fish which can go in a 15-30gal i can think of are;
Danios (zebra, leopard, pearl danio's etc- they're all hardy very active little shoaling fish)
Bloodfin tetras
Black neon tetras
Cherry barbs
Glolight tetras
Penguin tetras
White cloud mountain minnows
Black or White skirt tetra's
With small plecos, i have personally found clown plecos to be very robust little plecos and they grow to a small size however they do need driftwood/bogwood to eat and good filtration in the tank (their poop is like sawdust). Albino and peppered corys have also also been very reliable fish too in my experience.
To add colour and friendliness to the tank, you can't go wrong with common livebearers, however i would personally advise platys over guppys as platys seem to be hardier in my opinion- swordtails are also good though. The downfall with mixed gender groups of common livebearers is that they will produce fry, however there are many ways you can deal with fry. With swordtails, you can only keep one male in the tank unless you have a very large tank and an exceptional amount of females per male (like 15 or more), its best to either have 2-3females or more to a male, or just a trio or more of females and no males. If you opt for platys you can keep more than one male in the tank as long as you have 2-3females per male or have an all-male group of at least 5-7males or more, or you could just have an all-female group of platys etc.
The good thing in particular about fish like platys and swordtails is that they come in almost every colour and patterning imaginable so are great fish if you want friendly inquisitive and colourful fish in the tank- unlike a lot of other fish like danios and tetras, they actually notice you and will come to the front of the tank to great you excitedly etc
.Guppys are not as hardy as they used to be and tend to be riddled with diseases now days while mollys often end up needing slightly brackish tanks to survive and thrive in the long term, however if your friend opted for a slightly brackish tank then he wouldn't be able to keep any of the tetra's, danio's, minnows or catfish etc i have listed so far as they all don't cope well in salted tanks in the long term.
Gourami's are good for adding colour to tanks but they are not the hardiest of fish and you do need to take into consideration the genders of the gourami's you have as the males can be quite agressive towards each other and other fish- gourami's and fish like female betta's thrive best in heavily planted tanks (preferably with plants that grow all the way to the waters surface or has floating plants etc) with gentle current so would be ideal though if that is the sort of tank your friend is aiming for.
)Really p***** me of though, yesterday my poor moonlight gourami died, somehow there was not enough oxygen in the water, I saw a few fish gasping at the top, then him at the bottom, along with a little guppie 
