As Flame says, a bristlenose catfish needs a much bigger tank. It's not just that it grows to 12- 15 cms, but it is a massive pooper- I'm finding it hard work to keep my 63 gallon clean from 2 grown-up bristlies. It would simply be too much for the filtration system, and all the fish would suffer.
You can get a snail, or shrimps to clean the tank, but it is not necessary; you can also do your own cleaning.
A 5 gallon I would say is even a bit small for guppies- they are very active fish and like to poke around exploring. Endlers would be a better bet, they are that little bit smaller.
If you do go for guppies (not ideal), make sure you have the right ratio: 2 females would be ok on their own, but if you do males you need 5 to spread aggression. Mixed genders is going to cause fry problems (so might single females, actually), but if you can deal with that, the correct ratio is 2-3 females per male. 4-5 guppies would fill that tank though and not leave room for anything else.
Sex ratios are the same for endlers.
Neons are schooling fish, so they really want to be in a bigger group (minimum 6) to feel safe. A school of neons would fill the tank.
Please note that with any of those fish, you would need to do a fishless cycle (pinned topic); neons in particular are not hardy enough to cope with a new tank. You might prefer to look into other tetras, that are that little bit hardier.
If this 5 gallon tank were mine, I would first do a fishless cycle and then stock it with either 5 male endlers, or a single sparkling gourami+3-4 pygmy corydoras, or African dwarf frogs on their own, or a group of nice-looking shrimps.
